Tuesday, August 10, 2010

This is a Maria Theresa silver thaler whose shape would have given pleasure to Plato

Comments on Jean Nicod’s book “Geometry and Induction”

John Smythies

One of the most important contributors to the subject of the structure and geometry of visual space was the French logician Jean Nicod (1893-1924). The task that he set himself in this famous book was to derive the essentials of geometry, and the justifications for geometry, from our sensations (experiences) of the natural world, rather than from abstract reasoning. Since points do not occur in nature but volumes do, he interestingly based his approach on developments of the volume geometry of Alfred North Whitehead, and not on the point geometry of Euclid (pp 22f).

“ A ‘space’ is “any set of meanings satisfying the axioms of a geometry,,, We ask ourselves whether spaces exist. They do; we have come across one in the domain of numbers. But this does not interest us for we wish to see geometric order reflected, not in ideas but in sensible nature.” (p. 32).

Unfortunately, this book was written in 1922 and was based the naïve realist theory of perception years before the epistemological revolution in neuroscience proved that theory to be wrong (1). It has been established by experiment that we do not perceive the world as it is but as the brain computes it most probably to be (2). This fact has important implications for the logic of visual space.

Nicod claims “The elementary term and relations in nature are sense data. These are what we refer to as this, when we say to ourselves, in speaking of something immediately present to one of our senses, this is a tree, this is a penny, or again, this is a shooting star, this is the song of a nightingale.”(p.35)

However, what is “immediately present to one of our senses” is a physical object not a sense-datum. Secondly, we do not experience these physical objects directly: that which we experience is the phenomenal image of the external object created by the brain’s neurocomputational mechanisms. It is legitimate to base a geometry of the cosmos on the arrangement of the physical objects in that cosmos that we perceive via our sensations. It is also legitimate to base a geometry of our phenomenal world on the arrangement of the (usually visual) sensations that we experience by introspection. But it is not legitimate to confuse these two processes and attempt to base the geometry of the cosmos on the order of our sensations: nor is it legitimate to try to base the geometry of our sensations on the geometry of the external objects that these sensations represent. To help avoid this confusion we should not use the terms “sense-datum” and “sensum” (that derive from Russell, Broad and Price), as this tends to lead to confusion between object and representation: we should use the standard neurological term ‘sensation’ instead. Compare Nicod’s correct usage in—

“But at present I do not seek the most economical reconstruction of the order of the flux of my sensations. I am simply inspecting the relations I discover there. I try to apprehend each one of them as it presents itself to my mind.” (pp. 55-56).

Here he is clearly talking about his sensations.

with his incorrect usage in—

“The mind whose existence we are assuming is thus confronted with an infinity of distinct and simultaneous sense-data, corresponding term for term to the points in our physical space. This is the greatest possible intuitive perception of space.” (p. 107)

note also his statement

“Such are the elementary terms and relations of the sensible process. We have merely sought to present them before the mind’s eye. (p 66)

Note here also how Nicod says our sensations are presented to one’s “I” or to one’s “mind” or “mind’s eye”. This usage is currently unfashionable, but I think correct.


Nicod also lists some fundamental topological properties characteristic of the events that occur in a person’s field of vision (sic)—


He gives as an example the flight of an eagle seen by a person. During the eagle’s majestic glide it gives one flap of its huge wings—

Nicod says that the event ‘flap’ is interior to the flight.

However, here he is clearly primarily talking about the eagle as a physical object, not about his sensations of the eagle. (p. 37). The terms “field of vision” (or “stimulus field”) commonly refer to (A) the collection of external physical objects in range of vision at that moment. The term “visual field” refers to something quite different—to (B) the collection of visual sensations that we experience in phenomenal consciousness: B represents A.

Nicod says that interiority may also be experienced in somatic terms—e.g. by running a pencil over different fingers of one’s hand.

“This relation is distinct and obvious and springs to the [mind’s I hope] eye in each case.”


Thus ‘Interiority’ has both durational and extensional points of view. The squares of the physical chessboard are spatially internal to the rim of the physical board in physical space. Furthermore the squares that make up the phenomenal visual sensation of the chessboard in the visual field in consciousness are spatially internal to the visual image of the rim of the chess board: whereas the flap of the eagle’s wing isspatio-temporally internal to its flight in space-time (differentiated as above between physical space-time and phenomenal space-time). Nicod tends to confuse physical space and phenomenal space.


Nicod adds other basic topological relations in the same family as ‘interiority’——

‘Interpenetration’ (e,g, a row and a column in chess board)

‘Exteriority’ (e,g, 2 rows on chess board)

‘Continuum’ (touch but not penetrate: e.g. 2 runners in a relay race)

and equivalent temporal relations

‘Temporal inclusion’ (during)

‘Interference’ (overlapping)

‘Separation’ (not overlapping)

‘Prolongation’ (one starts as the other ends)

These same relations also apply to the visual field but in the different context of observable relations experienced between sensations rather than observable relations seen (perceived) between external physical objects.

Moreover, Nicod leaves out a large number of basic topological relations that are to be found in visual space (see further communications).


1. Smythies J.R. Ramachandran VS. 1998, An empirical refutation of the Direct Realist theory of perception. Inquiry, 40, 437-438.

2. Smythies J. 2009, "Philosophy, perception, and neuroscience" Perception 38(5) 638 – 651.


  1. To get a dialogue going, I thought I would make a few points. I have not read Nicod's book, but is sounds similar to the versions of naive realism defended by G. E. Moore in several works, and to some extent with J. L. Austin in Sense and Sensibilia. I completely agree that naive realism is false and that what we in common sense take to be the front surfaces of physical objects being seen, are in fact visual sensations in our conscious mind.
    Care needs to be taken with language used in trying to state other positions and there are dangers of being misconstrued no matter what terms are used. I have argued in my book, The Geometry of Vision and the Mind Body Problem, that ordinary language is theory laden (assuming the truth of naive realism) and that thus often violence has to be done to ordinary language just to state other positions. I agree that the statement what is "immediaately present to one of our senses" is problematic for more than one reason."Senses" here could either refer to our bodily sense systems or our sense experiences, but I don't see that in either sense that it is a physical object that is presented, since for the visual system, it would be light from the object that impacts the retina, and hence is the immediate physical stimulus. Using the word "sensation" rather than "sense datum" may be all right, but even here a word from ordinary language is used in a technical sense, and there are thus dangers of being misunderstood. What do people think about the term "qualia"?
    Robert French

  2. Alas, I think there are also problems with 'qualia'. This term, invented by philosophers, refers to "what it is like" to have a sensation, whereas what we are after is, I suggest, a term to indicate what it is to have a sensation. That is why, in my book "Analysis of Perception". I tried to define "sensation" ostensively by instructing the subject how to get an after-image and how to use the observable spatial (topological) relations between an after-image and the ordinary visual sensations around it, to define the latter.

    I quite agree that sensations are spatial particular existents located topologically in the conscious mind. I also hold that a visual sensation is a part of the person's organism and not part of an external object. When Lord Brain declaimed that "Externality is the cardinal problem" he was referring to the apparent fact that visual sensations (when we open our eyes) are simply "out there". But neurology shows that "out there" is a relation between visual sensations and the somatic sensations that make up the "body image", and that both of these are topologically inside the human organism. A major source of the present confusion over the mind-brain relationship is the widespread confusion between the body image and the physical body.

  3. This business of being "out there" and externality of vision may tie in with what Berkeley par. 46 of his New Theory of Vision calls "outness" which seems to refer to a sidedness of our visual experience whereby we experience phenomenal visual space as being "out" away from us, although it is still just a part of our consciousness.

  4. My reponse will be in two parts, of which this is Pt. 1:

    Many years ago when I was in the graduate program of philosophy at UCSD, that which some recent philosophers disparagingly call "folk psychology" (notably Paul Churchland in UCSD philosophy at the time) was discussed in a seminar on the philosophy of mathematics. One of the graduate students objected that so-called folk psychology did not constitute a viable theory opposed to the theory of scientific psychology for the simple reason that folk psychology is not a theory, had never been propounded as such but, if anything, rests largely on observations and ideas based on everyday experience. It is thus neither a philosophical nor a scientific theory because it does not exist in a form that is falsifiable. Rather Francis Crick once said in response to some scientific notion that had been advanced, “It is not even wrong!” In his remarks on Frazer’s Golden Bough, Wittgenstein raised a very similar objection to the “explaining away” of magical practices and religious beliefs in “primitive” cultures by folklorists, ethnographers, and anthropologists, taking issue with Frazer’s contention that such beliefs are due to stupidity, and that they are just “errors” or “mistakes.” He writes: “Frazer’s account of the magical and religious notions of men is unsatisfactory: it makes these notions appear as mistakes. Was Augustine mistaken, then, when he called on God on every page of the Confessions? Well—one might say—if he was not mistaken, then the Buddhist holy-man, or some other, whose religion expresses quite different notions, surely was. But none of them was making a mistake except where he was putting forward a theory.” Of course, the whole point is that none of them were really putting forward a theory, especially to the extent that their beliefs rested on some form of faith.

    I would propose that it may be the same with so-called “naïve realism,” and much like folk psychology, magic, and religious beliefs, it is not really a theory, certainly not in any rigorous sense, but a pseudo position that philosophers formulated solely for the purpose of refuting, rather like the proverbial straw man. Like the stupidity claimed of primitive beliefs, its naivety stems not from any false theoretical premise (let alone everyday experience), but from a mistaken conception on the part of the philosophers who formulated it and who, in the process, inadvertently fell into the trap of comparing apples and oranges.

    The basis of that mistaken conception is the subject of an unpublished monograph of mine, The Localization of the Mind: Why We Think the Mind is in the Head, in which I presented a conceptual analysis elaborating certain points made by John Smythies in his Analysis of Perception, namely, that there are two systems of spatial relations involved in the study of perception and cognition, and that they are commonly (and perpetually) confused, even by scientists and philosophers: One system is perceptual, the other physical. The perceptual system consists of spatial relations within the perceptual world between perceptions and mental images. The physical system consists of relations within the physical world, including the human body, but does not include perceptions and mental imagery. In addition there is a (putative) relation between the two systems, i.e., between the physical world (and body) and the perceptual world (and body) as studied, for example, by psychophysics. Often these two systems of spatial relations are a source of confusion because they are combined in various ways rather than being distinguished from each other. The confusion can be eliminated by keeping them separate.

  5. This is Pt. 2:

    The source of the confusion arises from the fact that the act of perceiving is itself contained and perceived (or at least partly so) within the perceptual world. For example, someone looks at a tree outdoors. The act of looking with one’s eyes and the tree being looked at are both perceptions. So when the “naïve” observer observes a tree, or its surface, he is correct, not in error, in saying that he is seeing an object and its surface. He is seeing a visual object and its surface—just not a physical object and surface. The process of observing and the object of observation are both within the perceptual world, and are not something happening between the observer and the physical world. Both the eyes gazing and the tree are objects in the perceptual world. An observer is therefore not mistaken to the extent that what he reports are relations within the perceptual world which, however, is commonly called the physical world as well, something that only adds to the confusion.

    As both Moore and Wittgenstein were at pains to show, ordinary language makes it very difficult to go afoul if one heeds its limitations, because ordinary language contains something like a picture of the (perceptual) world, doubtless because language developed within the (perceptual) world itself.

    So I would not agree with Bob French that ordinary language is “theory laden” at all, for the simple reason that the perceptual world is not a theory but perhaps the most supremely real thing we all know, and know best: Our immediate experience. The rationale for formulating so-called “naïve realism” as if it were a theory needs to be examined more carefully for the reasons I have stated.

    Similarly the history of the distinction between primary and secondary qualities (Locke), the whole notion of “sensations” and “sense impressions,” and more recently, “qualia,” has been the topic of a fairly rigorous analysis by philosopher Peter Hacker, “Is There Anything it is Like to be a Bat?” that appeared in Philosophy (The Journal of the Royal Philosophical Society) in 2002, but can be accessed at http://www.jp.philo.at/texte/HackerP1.pdf
    He gave a much simplified version of the argument he presented there in Philosophical Foundations of Neuroscience (pp. 271-292). I am unaware of any challenging rejoinders to his argument. As Hacker wrote to me himself, the problem (or pseudo problem) of qualia are “no more than a philosopher’s opium dream.”

  6. Mea culpa! In rereading Hacker's paper, I find that I am mistaken in saying that it reviews the idea of sensations and the Lockean dichotomy twist primary and secondary qualities. Rather, qualia as he understands the usage among philosophers, and as adopted by some neuroscientists, are much more general attributes of consciousness than the simple qualities we call colors, sounds, touches, smells, etc., but are more associated with global aspects of consciousness and subjectivity. What Hacker does not note, though, is that the qualia theorists have unwittingly rediscovered a whole class of perceptual phenomena that were extensively studied by Ganzheit ("totality") psychology in the 1920s and early 1930s, which was a sister discipline to Gestalt theory, founded by Wundt's successor at Leipzig, Felix Krueger. It was a holistic approach to perception that studied whole properties of experience rather than piecemeal, as most research on perception still does. Like Gestalt theory, their point of departure was the theory of Gestalt qualities advanced by Christian von Ehrenfels. Stated simply by Hans Cornelius, the idea was that feelings (in the sense of the "feeling of a place" for example) are the Gestalt qualities of a whole experiences, not just, say, some given shape or configuration, maintaining that Gestalt (configuration) was a special case of Ganzheit, and that Ganzheit phenomena are more fundamental to perception in the scheme of human development. Needless to say, this would be relevant to the study of visual (or perceptual) space as a whole, i.e., whole properties of it.

  7. First, a comment on Bill Rosar parts 1 and 2

    I fully agree with most of what Bill Rosar says, but I am uneasy about his use of the term 'perceptual system' instead of the usage many people follow, which is the 'phenomenal system'.

    In his posting he says—

    "So when the “naïve” observer observes a tree, or its surface, he is correct, not in error, in saying that he is seeing an object and its surface. He is seeing a visual object and its surface—just not a physical object and surface. The process of observing and the object of observation are both within the perceptual world, and are not something happening between the observer and the physical world. Both the eyes gazing and the tree are objects in the perceptual world. An observer is therefore not mistaken to the extent that what he reports are relations within the perceptual world which, however, is commonly called the physical world as well, something that only adds to the confusion."

    In this Rosar refers to the "physical world" and the "perceptual world" as basic entities. This, I suggest, might run the risk of appearing to some as defending a Wittgensteinian defence of the naive realism of 'ordinary usage'. Whether naive realism is, or is not, a theory, surely, as Bob French claims, there is a mountain of evidence that shows that it is absolutely wrong. So, in order to avoid appearing to support it in any way, I suggest it might be better to stick to the terms 'physical world' and 'phenomenal world'. In that case we can say that we see trees, etc. which are physical objects. Then, as a later step in the whole extended process of visual perception, we can say that we experience visual sensations. Visual perception starts in the physical world in the form of light rays bouncing off physical objects, and ends as visual sensations experienced in a person's phenomenal visual field. We do not see visual objects—we see common everyday physical objects. Nor do we see the phenomenal visual objects, constructed from individual sensations, in our visual fields (in the manner extensively described by Paul Schilder and others)—we experience these.
    A short comment about whether visual phenomenal space is two- or three-dimensional. If we consider the basic visual field we experience in the complete dark, it appears to be a 2-dimensional blackness: yet I seem to be set back from it and can direct my attention differentially to sundry events in it—like two fading after-images, or the usual faint blue swirling clouds— and I do not seem to be IN the 2D blackness itself. Nor do I feel myself to be IN the ordinary visual field we experience with our eyes open.
    But the black field is not always 2-dimensional. In sensory deprivation experiments, John Lilly reported that in several occasions the 2-D black field suddenly opened before him up to form a 3-D space, which he found very intriguing and eagerly awaited to see what happened next.

  8. Again, part 1 of a 2-part reply to John Smythies:

    The standard term in the scientific study of perception for what philosophers choose to call "phenomenal space" is indeed perceptual space, and it is a term that has been used now for about a century in experimental psychology. That part of it which is visual is called visual space. Perceptual space does not contain anything physical, even if it is apparently causally linked to physical processes (at least according to the casual theory of perception, which presupposes classical causality).

    In my opinion Moore, Broad, and Wittgenstein were all correct regarding ordinary language (as I will explain in my forthcoming commentary on Nicod) to the extent that it referred to the perceptual world, not to the physical world as defined by physical science, which they left to scientists to explain (most critics of their work fail to notice that important caveat).

    Naive realism is, as I said, not a bonafide philosophical position at all, but something concocted by philosophers to be false. To propose a theory merely to refute it is a straw man argument, unless the theory has truly nothing to recommend it--which seems absurd at the outset. Therefore refuting it is meaningless, because as stated by philosophers, it is inherently untenable.

    Obviously since we take the perceptual world as reality, to simply dismiss it out of hand is to saw off the limb we are sitting on, and perhaps amounts to refuting reality altogether, which in the case of the perceptual world, is apparently constructed in some way (as even Plato seemed to realize), much as the physical universe is constructed in its way according to the laws of physics. So there is a *defect* both in the notion of naive realism as a "theory" and therefore in the arguments against it, as I have tried to explain above already. Somehow I don't think that got across.

    It follows that what is being called its "refutation" by Smythies and French (and apparently Ramachandran) is more akin to what Einstein was doing with relativity to the extent that multiple observation points had to be taken into account. He was seeking what is *observer invariant* in the general theory of relativity rather than what was observer independent. That is one reason why Bernard Carr is at pains to account for all possible perceptual vantage points in his theory of the Universal Structure (I'll let him explain that here if he so chooses).

  9. Pt. 2 of another 2-part comment:

    The causal sequence that Smythies gives of visual perception that includes physical processes is something taking place in the (putative) physical world, not in the perceptual world. What I described was perception as experienced *wholly in the perceptual world,* i.e., mainly the visual part of it, but also kinesthetic in perceiving our own eye movements as we direct our gaze at an object (a *visual* object, not a physical one it should be noted). There is nothing physical in the visual world at all, otherwise one is guilty of smuggling so-called "naive" realism in through the back door, and defeating the whole line of argument against it.

    All of us are constantly guilty each day of our lives of lapsing into naive realism, if that consists of believing that what we experience is physical reality as defined by physics. Yet the problem is not so easily stated, because as Mach observed over 100 years ago, physicists construct "pictures" of the physical world that we only experience through perception (or mental imagery), thus again within the perceptual world, or world of imagination, which is closely linked to it. Schroedinger was thus wrong in his "Mystery of the Sensual Qualities" that the physical picture of the world is devoid of sensations (as was Eddington in his second of the two writing tables), a point forcibly argued by Susan Stebbing in her "Furniture of the Earth." Were it otherwise, we would have no "picture" of anything physical at all. Everything we experience is in some way way sensation-based, or sensation derived--even mathematical computations--and by that I mean the contents of the perceptual world and mental imagery.

    What Smythies describes above with regard to black visual fields, sensory deprivation, his own viewpoint as an observer etc., is ostensibly all occurring within the perceptual world known only to percipients, not from an Archimedean point outside it. Ergo, even the observer himself is a perception--which is perhaps the weirdest realization of all--a point that Descartes apparently missed in trying to remove his thinking self from the sensate world of perception and imagination.

  10. To make a few points in reply to comments, in saying that the ordinary language of perception is "theory-laden" I do not necessarily mean that it assumes the existence of any well developed theory, but rather that it has hidden assumptions, which at least can be questioned. I take at least three of these assumptions to be at least roughtly given as 1. We are immediately aware of the front surfaces of physical objects when we look at them,2. what we are immediately aware of here is physical (in some sense which is not compatible with just being part of our conscious mind) and 3. that these objects (as immediately perceived)continue to exist in the same format when not perceived.I think that each of these assumptions is clearly false (I think that John agrees with me here and I am less sure of Bill). It pays to bring points of disagreement into the open so that they can be independently discussed.
    I don't think that I am beating a dead horse here since I have heard noted philosophers, such as Hilary Putnam for example, describe themselves as being naive realists. An interesting question is also whether there is a difference between direct realism (assuming that there is just one position here) and naive realism, and, if so, where exactly they differ.
    One way I think that just analyzing ordinary language is seriously misleading in the philosophy of visual perception, is that verbs, such as 'see' are transitive and thus assume a distinction between the object seen and the seer. Perhaps this is why some people have complained about talking about "seeing hallucinations."
    I also think more needs to be done on the issue of whether visual space is 2D or 3D. I don't think that Berkeley's point that we experience the space as being "out" away from us entails that it is 3D. I have also heard the example of perceiving semitransparent objects given as a counterexample to my position that it is 2D but I am not convinced.

  11. If naive realism is not a real theory (if even a theory) as I am arguing here, then obviously any ordinary language which has been co-opted by philosophers in support of it has been misappropriated (as it were). Particularly with Moore linguistic philosophers are notoriously perverse in forcing the issue to show that ordinary language really does work for a surprisingly large number of philosophical problems, if one simply adheres to it, and does not do violence to it.

    That said, ordinary language is not intended for scientific use, and therein lies the crux of the matter. When anyone says they "see" or are "immediately aware" of objects (as Moore would do with his hand and foot), he is only referring to objects in the perceptual world and is not making a scientific claim about the nature of physical reality as studied by physics.

    Without knowing the context in which Hilary Putnam claims to be a naive realist, I cannot comment. But otherwise I have never known of a philosopher who expressly claims to adhere to naive realism. With so-called "direct realism" it would be necessary to understand what exactly is meant by "direct."

    But isn't all of this rather afield of articulating what is known--and perhaps what can be known--about the structural nature of visual space, which is the object of this forum?

    In a sense Hans Reichenbach with his "coordinative definitions" was reverse engineering geometry with respect to the physical world, much like Nicod was with the perceptual world, in showing how it grows out of perceptual experience. This is why Einstein referred to "physical geometry" rather than just "geometry." So whether binocular visual space is a 2-D surface (or volume as Nicod suggests) depends upon how dimensionality is being defined, what criteria are stipulated in its definition, etc.

  12. I agree Bill, it might be more productive to stick to issues about geometry. In particular, I think it would help to become clearer about the issues with dimensionality. Do you know whether Nicod gives any reasons for thinking that visual experience consists of volumes (which sounds 3D) as opposed to 2D surfaces?

  13. Truth to tell, I have not read Nicod's book, and probably should have, given what Smythies has synopsized of its contents above. Since Smythies is the one who seems to know the book he might be the best one to answer your question.

    Dimensionality is considered by cosmology to be both a topological and metric property of physical space. But if Nicod is right, the origin of the very idea of dimensionality is to be found in visual experience. This then leads logically to the question: If visual space is two dimensional, how did it give rise to the idea of three dimensions, perhaps more fundamentally, of spatial volume? If Nicod did not do that, we need to de-construct the idea of tridimensionality back into its perceptual antecedents.

    As Avrum Stroll showed in his book on surfaces, ordinary language is a very good tool for that purpose. Unfortunately Stroll made the error of comparing scientific findings about physical surfaces with the ordinary language used to describe perceptual ones, which rather muddied the waters, and resulted in a classic contradiction in terms (surfaces in the physical world are not really surfaces at all). That just goes to show that ordinary language refers to the perceptual world, not the physical one characterized by physics.

  14. Reply to Bob's question re Nicod.
    The reason he gives is that he prefers Whitehead's geometry to Euclid's because the former is based on "real entities" found in Nature (volumes) and not on Euclid's points and lines, which are not found in nature.
    Bob's second point about getting back to geometry: before we do that we need to be quite clear about what structure this geometry belongs to. Next posting (in two parts) is about that.

  15. Part 1
    Perceptual space or phenomenal space?
    The discussion so far seems to indicate that we need to agree on unambiguous terms to define and describe the varieties of space we are interested in. We may agree that there is one space (A)—in which (a) a person’s visual sensations, after-images, eidetic images, ordinary visual images and related phenomena, and visual hallucinations are both located and extended: and (b) in another locus of the same overall space—in which a parson’s somatic sensations (that make up the neurological body-image) and somatosensory hallucinations (e.g. phantom limbs) are both located and extended. We may also agree that there is, in addition, another space (B), in which physical objects such as protons, brains and stars are both located and extended. Then we need to find names for (A) and (B). (B) is surely physical space? (Note: not the mathematics used to describe the geometrical properties of this space, but the space itself). So what is the best term for (A)?

    What do the fonts of all wisdom (a.k.a. Google & PubMed) suggest?
    A search (1984-2010) revealed 9 papers with “phenomenal space” and 18 papers with “perceptual space” in their title. There were also 2 papers with a confusing mixture of both: (details available as an appendix on request). So your pays your money and takes your choice. I am quite happy with the term “perceptual space” so long as this adheres to the definition A given above. The intrinsic geometries of A and B are different problems tackled by different methods. Moreover, the geometry of the relationship between A and B is a third topic of its own.

  16. Part 2
    The visual field.
    If we next turn to what terms visual neuroscientists use in this context we find that they mostly talk about the “visual field”. However, further investigation shows that they can mean by this term, in different contexts, both the stimulus field (spatial system B) and the phenomenal field in consciousness (spatial system A). For example, Francis Crick says in his book “The Astonishing Hypothesis” that ‘stimulus field’ (space B) and ‘visual field’ (space A by context) are synonyms. I pointed out in my reviews of this book in “Brain” and “Inquiry” that to say that A and B are the same leads to the absurd conclusion that the input to a computational system is the same as the output. For surely any neuroscientist has to accept that system A represents the output, and not the input, of a vastly complicated neurocomputational system?

  17. A puzzling neurological aspect of the visual field and its geometry.

    The visual experiences of blind people with retinal lesions and occipital lobe lesions are quite different. A person without functioning eyes but with a normal brain experiences the world as we do when we are in the complete dark. This person still has a geometrical visual field even though it is always a featureless black expanse. We can say that his/her phenomenal visual space still exists. They can see nothing of the world around them but they still have visual experiences even if useless and dull. In contrast, people with severe enough lesions of the occipital cortex do not experience any such uniform black field—visually they experience nothing at all. Such a state of affairs can be obtained temporarily in normal people in retinal image stabilization experiments: they too report that they see nothing at all—which they usually find terrifying. All visual geometry has vanished. Thus do we agree with the neurologist Jason Brown that we have to conclude that the occipital lobe’s activity actually creates visual space and its geometry? If so, how does it do it? How do neurons create real space?

  18. I rather have assumed we were all talking about the same space mainly, whether one calls it "phenomenal space" as philosophers tend to do, or "perceptual space" as psychologists studying perception have done for many decades (especially those working in the field of psychophysics). I prefer "perceptual space" or, more specifically, "visual space" or "auditory space," because they are more descriptive.

    I cannot fathom why Crick (R.I.P.) identified the visual field with the stimulus field, and would need further details to offer an opinion.

    But on the basis of what you say about Jason Brown's views, I would say that to assert that the "occipital lobe's activity actually creates visual space and its geometry" would be jumping to conclusions, merely because the two are linked causally. If anything, what we know of the visual cortex now suggests that it is decomposing the visual input from the eyes in various ways rather than creating anything like the visual world that we experience. I would even go so far as to say that the brain appears to be decomposing the visual world we experience! To me this suggests that there is a fundamental flaw in our causal reasoning about perception, and because of that I can understand why James Culbertson proposed that sensations occur *before* brain processes do. The problem with Culbertson's view is that he could not produce a physical/neurophysiological model for this that worked scientifically, though physicist Nick Herbert offered his own "version" of Culbertson's theory in the book Elemental Mind. But then, the science may be wrong, or at least incomplete.

    But John, you have not answered Bob's question as to wheteher Nicod says anything anout *dimensionality*. Does he?

  19. Yes, space "B" above would be physical space, as distinguished from space "A" (perceptual space/phenomenal space). But I say that more for purposes of analysis than for ontological reasons. It is best to hypostasize two spaces, even though British psychologist Michael Morgan seems to think it a stupid idea (for reasons that are not very clear to me). But eyes and brains also exist in perceptual space to the extent that they are perceived in the first pArson [sic!] subjective (we see ours eyes reflected in a mirror, or feel them in our eye sockets) and we can view other people's brains in surgery or pathology or medical exhibits, as in the UCSD Center for Brain and Cognition. The physical counterparts of these things are perhaps much more mysterious. It seems to me that the physical world revealed by physics is much spookier than any spook in parapsychology (pace, CSICOP!)

  20. I prefer "phenomenal space" over "perceptual space" since I think that there are fewer opportunities for misconstruals into something physical in the brain. A and B are also fine,so long as people are clear about the definitions.I know that in Part 4 of Process and Reality (which came out in 1929, after Nicod was writing), Whitehead gives a theory of geometrical elements in terms of abstractive sets, where something without magnitude, such as a point, is seen as a limiting case of smaller and smaller things with magnitude, and it sounds like Nicod was sympathetic with the concept. Do you know if Nicod was also sympathetic with Whitehead's panpsychism? If so, this might explain why he thought that experiences are 3D since he would then identify the phenomenal with the physical, and since the physical is 3D.

  21. The reason I do not prefer "phenomenal space" in English usage is because it is predicated on "phenomena," which can be either physical or perceptual. Because the major advances in the study of visual perception have mostly been made by science in the past 150 years, whether by experimental psychologists, sensory neurophysiologists, ophthamologists, or even neurologists, in the case of vision I think we should use the term mainly used in that literature, namely, "visual space," which has been used since the mid-1800s by scientists and some philosophers as well.

    As a more general term, collectively denoting the spatial aspects of all the senses, "perceptual space" was being used by science at least since the 1930s, and was already used by philosophers in the late 19th century (a quick Google Books search revealed that much). Phenomenal space, though used in the 19th century, seems to have been a term used mainly for purposes of philosophical discourse, and is more ambiguous in terms of its referent, as should be evident from this philosophical text, Exploratio philosophica: rough notes on modern intellectual science, Part 1 By John Grote (published in 1865): http://books.google.com/books?id=DlUuAAAAYAAJ&pg=PA26&dq=%22phenomenal+space%22&hl=en&ei=_Q10TMPJA4zSsAPkzM2oCA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CC0Q6AEwAQ#v=onepage&q=%22phenomenal%20space%22&f=false

  22. Post in 2 parts
    Part 1.
    1. Seems to me that people on this blog should feel free to use what term they want—be it "phenomenal space", "'perceptual space" or "visual (or somatosensory) space". I do not think this would lead to any misunderstandings.
    2. Francis, as he frequently emphasized, had no use for philosophers. Therefore it is not surprising that he should make philosophical mistakes. He did not realise that to move from his favourite neurophysiology to the problem of mind entails crossing a minefield set with a large number of metaphysical booby traps. I once had a long discussion with the very savy Vernon Mountcastle at Milan Airport about the perils of reductionism. Yet even he did not see that facts about how the brain works belong to a different logical category to the question of how these brain events are related to the events we experience in consciousness—as pointed out amongst others by Freddie Ayer.

  23. Part 2
    3. I looked through Nicod again to check on Bob's Q about dimensionality. Nicod says very little about this, as he concentrates on topological ideas like 'between', 'inside', 'outside’, etc. However, I found one passage of possible interest on p. 97 where he says that
    “…any translations whatever of his [the observer’s] body from the reference position correspond to the homogenous double alternations which have just been defined…..Geometry is thus wholly expressed in this EXPERIENCE. Homogenous double alternations and the connection of their pairs provide our subject with an interpretation of points and congruence and congruence in every statement geometry makes about them; and we know that a full expression of geometry can be given in terms of these expressions alone.”
    This seems to claim that the subject’s EXPERIENCE of the 3-dimensional rotations of his own 3-D body can supply all that geometry needs. Thus, presumably, the somatosensory space in conscious experience (that contains the neurological body-image) at least must be 3D. But this does not relate necessarily, if at all, to visual space.
    I doubt that Jean Nicod was interested in pan-psychism—but who knows!
    4. I do not agree with Jason Brown either. More research, it seems to me is needed on the question of what cortically blind people actually experience.
    5. I like Bill’s idea that “ the brain appears to be decomposing the visual world we experience.” Certainly if you tweak the 2A serotonin receptors with LSD, you get the perception of a more highly composed visual world than usual—straight, as Jung agreed in a letter he wrote to me in 1952, from the Collective Unconscious.

  24. Further to the idea that the brain may be decomposing visual space, as much as “causing” it, I would maintain that most vision science today (including neuroscience related to it) is guilty of “naïve” realism, in that the visual displays as seen in a subject’s visual space are what is being “seen,” not physical objects as described by physics. This applies to those performing the experiments and the experimental subjects alike. The same is probably true of most research on perception in any sensory modality being carried out by cognitive science on the whole, which naively talks about visual displays, whether computer-generated or “real” world ones as if they are that which is being “looked at” and “seen,” rather than the sort of mysterious object that Eddington described as his second writing table, that consists mostly of empty space. Surely such an Eddingtonian physical “object” is not what the experimenters or subjects believe is being seen, but rather, just what they ordinarily see with normal or corrected vision. Again, this refers to relations of seeing and objects *within* the visual world, not to anything physical.

    The visual world is obviously what gives rise to the idea of there existing physical objects that are independent of vision, that *look like* them as well. But the visual space we have been discussing thus far is at best known only through foveal vision, as it is very difficult to perceive angles outside the foveal range, and virtually impossible in peripheral vision. But again, this refers to things *within* visual space, not outside it. Thanks to foveal vision we have built up an *idea* that there is a visual world that has the resolution characteristic of foveal acuity. Even someone whose vision has been severely impaired due to injury or disease still believes that there is a world “out there,” only that they can’t “see” it. But that world is the visual world, which at any given instant, contains only a small region of foveal resolution.

    Any takers?

  25. Since eminent physicists like John Archibald Wheeler and Eugene Wigner are somehow given permission to voice veritable flights of fancy about science, philosophy (cf. Wigner’s “Remarks on the Mind-Body Problem” in the volume edited by I.J. Good entitled, “The Scientist Speculates”), the cosmos, and observation, why can't those who work in the study of perception and cognition as they relate to the brain do the same about physics and cosmology? Crick seemed to be virtually shackled by the fallacies of neuroreductionism (as roundly and brilliantly critiqued by William Uttal already in the 1980s), except when it came to the daft reasoning of "Directed Panspermia" (his pet crank interest, it seems).

    Half jesting I suggested to Rama recently that perhaps the brain is an epiphenomenon of mind, and he liked the idea sufficiently that he is quoting me in his new book. But if taken seriously in the context of this discussion, I do not believe that entropy alone (for example) would make it impossible to run the causal sequence of creation in reverse, so that what science believes is the universe moving forward in time, is actually moving backwards towards the other end of time, perhaps created by (something like) a mind, recalling astronomer Sir James Jeans and his famous remark (1931) that “The Universe is more like a great thought than a great machine.”

    That would eliminate the problem of retrocausality, or alternatively, might recommend the notion that creation is reciprocally created from either end of its existence, using Bernard Carr’s image of the uroboros swallowing its tail. So when Culbertson suggested that sensations are occurring before brain processes, he may have been in some sense right, which would explain why the brain appears to be decomposing visual space (for one) rather than constructing it, as most neuroscientists studying visual neurophysiology NAIVELY suppose. It is as if we need to turn the whole causal sequence inside out to make sense of it all (if I may be forgiven a topological metaphor!)

  26. I thought I might say a few things regarding Nicod's suggestion of tie-ins with somatic sensations regarding the issue of dimensionality. Berkeley, for example held that while visual experiences are 2D, touch is 3D and connections between the two are like translations between different languages. I don't know as I am convinced though. There is a vague sense of direction with somatic sensations (and also auditory ones) which introspectively seem to be coordinated with visual space, such as if one pinches one's finger while looking at it, or looks at a source of sound. The issue of bodily motion may also be key for learning that the physical world is 3D, such as by reaching for a seen object and not being able to touch it, or walking around an object. More needs to be said here about assumptions of naive realism throughout this process, since that, at least sometimes, identifies visual space with physical space, or at least elements of it.

  27. I think that Nicod was referring to proprioception rather than touch. Also we can avoid slipping into naive realism if, following John Searle and Paul Schilder inter many al, we keep the key distinction between the body-image and the physical body clear in our minds at all times.

  28. My comments on Nicod will be piecemeal, which should perhaps be better for purposes of discussion anyway.

    What Nicod seems to have attempted is along the lines of what philosophers have since his time called “recovering a definition,” in this instance, defining geometry in relationship to visual experience rather than as an abstract formal system of axioms involving deductive reasoning. In this regard Nicod’s aim paralleled somewhat that of the reform of language movement in philosophy, which argued that everyday discourse is perfectly suitable for (most) philosophical purposes: “[T]he mischief lies in deviating from ordinary language without providing any way to make sense of the deviation,” notes the Encyclopedia of Philosophy entry on the Philosophy of Language. Furthermore, “Wittgenstein seems to have held that all or most of the problems of philosophy arise from the fact that philosophers have misused certain key terms such as ‘know,’ ‘see,’ ‘free,’ and ‘reason.’ It is because philosophers have departed from the ordinary uses of these terms that they have become entangled in insoluble puzzles over whether we can know what other people are thinking and feeling, whether we ever really see physical objects, whether anyone ever does anything freely, and whether we ever have any reason for supposing that one thing rather than another will happen in the future. The proper role of a philosopher is that of a therapist. He must help us, the perplexed, to see the steps by which we have unwittingly slipped from sense into nonsense; he must lead us back to the ordinary use of these words, on which their intelligibility depends, thus relieving the conceptual cramps into which we have fallen.”

    Alas, Nicod died prematurely (d. 1924, age 31) before the reform of language movement in philosophy really flourished, most notably with the work of Moore, Charlie Broad, and the later Wittgenstein in Britain. But in effect Nicod was seeking the “primitives” (elements) of geometry in visual experience, much as the ordinary language philosopher sees in ordinary language something like a “picture” of the (perceptual) world. To that end Nicod’s goal with geometry involved a “recovery” of the most basic visual elements upon which geometry was originally developed. For example, he clearly recognized that the idealized extensionless point of geometry is not something that can be seen, and yet the word “point” in ordinary language refers to something than most certainly *can* be seen, for example, a period such as ends this sentence. On the other hand, the geometrician reduces the radius of a point to zero, so that it is no longer something visible, which at the same time removes it from the world of visual experience and perception—and from the *sense* of “point” in ordinary language. Thus a simple example of the “nonsense” which technical language makes of ordinary words.

  29. I find it impossible to understand Nicod’s statement that John quotes: “…any translations whatever of his [the observer’s] body from the reference position correspond to the homogenous double alternations which have just been defined…..Geometry is thus wholly expressed in this EXPERIENCE.” Without being told how Nicod is defining “double alternations” it is impossible to evaluate his claim that “geometry is wholly expressed in this EXPERIENCE.” He does not define in what sense he is using the term “geometry” here either, nor “EXPERIENCE,” and I suspect his claim is undetermined by visual experience, because unless “double alternations” refer to something in visual space, some sort of reasoning must be involved above and beyond what is visually given (it is not clear whether he means the *visual* perception of one’s body, or somatic as well.)

  30. The assault on "naive" realism was already well underway in 19th century philosophy, and did not require the findings of neuroscience. Cf. “A brief history of modern philosophy” by Harald Høffding, published in 1922 (the same year Nicod's book was published): http://books.google.com/books?id=pZYlACTkc74C&pg=PA282&dq=perception+%22naive+realism%22&hl=en&ei=gGx1TIHPAoH6sAOsuNmgDQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCYQ6AEwAA#v=onepage&q=perception%20%22naive%20realism%22&f=false
    It should be noted, though, that in philosophy, the notion of “naïve realism” is much broader and does not refer solely to perception. My guess (and it is only a guess) is that the notion of naïve realism is a result of the Enlightenment. There is yet much to be learned from the history of philosophy.

  31. The assault on naive realism is even present in Descartes, but it also periodically gets revived, particularly in the twentieth century, for example with the work of J. L. Austin. I at least have run into a lot people who claim to be direct realists, and I am not at all clear on how, if at all, this is supposed to differ from naive realism. But, if we can agree that naive realism is false, perhaps we should concentrate on the geometry of visual space.

  32. The problem with naive realism as a (pseudo-)philosophical position is that it is formulated to be false, rather than to be falsifiable, which means that it is invariably stated in such a way that it is only false, never true. Therefore assenting to it being false may accomplish little, and may make it difficult to maintain realism of any stripe as a consequence, simply because the basis of realism is (ostensibly) perception. So discarding the "naive" version may be rather like throwing out the proverbial baby with the proverbial wash, if all notions of reality are derived from perception. Another way of putting it is that it may be like sawing off the limb upon which one is sitting--either quaint saying applies.

    It is not clear whether so-called "critical realism," which 19th century philosophy apparently offered as a sort of antidote to the naive form, is any less mistaken (or false), with its appeal to "scientific objectivity" as a corrective measure, because scientists so frequently lapse into different forms of naive realism themselves (if unwittingly).

    As nearly as I can determine, most scientific research on perception is ostensibly founded on a kind of *un*critical realism—that is, a sort of “third person naïve realism,” as I have been calling it these days, and which I have been at pains to explain already. If that is so, then the problem is perhaps less with space "A" as defined by John, than it is with space "B," because we may simply be talking about spatial (let alone geometrical) relations within the visual world without realizing it. I don't see this as being irrelevant to our task at hand, because it may be less obvious than we think where the perceptual world leaves off and the physical world begins. Merely hypostasizing two different spaces may not be sufficient in the final analysis.

    For example, as nearly as I can determine the role of observation in relativity theory, at least as described and defined by Einstein himself, makes no distinction between naive realism and critical realism. It merely talks about different observation points, because of Einstein's realization that there is no Archimedean point outside the universe from which to observe it, only an infinitude of observation points within it, the only upper limit constraint being the speed of light.

    In any case, if contemporary physical science makes no distinction between naive realism and critical realism, we have a serious epistemological problem on our hands!

  33. For a concise late 19th century statement of so-called "critical realism" as distinguished from its naive counterpart, see the textbook by structural psychologist Oswald Külpe, "Introduction to philosophy: a handbook for students of psychology, logic, ethics, æsthetics and general philosophy," translated as it was by fellow disciples of Wundt, Edward Titchener and Walter Bowers Pillsbury (both Cornell men):

  34. If I may throw in one more quaint saying, as it were an apology for "naive" realism, "Seeing is believing." It follows that seeing that a line is straight, seeing that a shape is square, circular, or triangular, is in the visual world tantamount to saying those things are de facto straight, square, circular, or triangular. Where does one go from there, Bob?

  35. I'm am also wondering at this point (no pun intended) how important and necessary it is to review Nicod's ideas? I would be interested to know what John has found most relevant in them, and then discuss that, because there are quite a few potential points of contention (not extensionless ones) in that precis that may prove a waste of time to debate.

    We need some sort of focus rather than just the "geometry" of visual space (whatever exactly that proves to be, given that efforts thus far to geometrize it may rest of erronenous assumptions, problems with measurement, and problems with interpretation).

    Secondly, can the question of the topological/geometrical nature of visual space be disentangled from questions of realism and the mind/brain problem or not?

  36. You are right Bill, there are a lot of interconnected issues here, and thus it is possible to go off on different tangents. My own position is that phenomenal visual space is a distinct space from physical space although they are causally linked in the brain. I also think that our knowledge of the physical world is pretty indirect, although in the case of the perception of macroscopic objects, aside from illusions, the attitude of naive realism where we identify the two spaces, works reasonably well. There is a nice quote from Bertrand Russell, in Inquiry into Meaning and Truth, where he states that naive realism leads to science, which in turn shows that naive realism is false, which shows that naive realism, if true is false, and therefore is false. I pretty much agree. The world of physics is very different from the way we perceive it, but I do remain a physical realist - see my website www.quantumrealism.net.

  37. Merely agreeing with Bertrand Russell et alia about the falsity of naïve realism doesn’t get us very far, I fear. Perhaps I should say at this point that the view I am developing is both post-naïve realism critique (as it were) and post-representationalism. My reasons for being “post” hopefully shall become apparent, because I now believe both positions are inadequate to explain the facts, even as we already know them.

    Bernard Carr, who as John and I know is a mathematical astrophysicist, apologizes for his delay in joining the discussion, but is currently just finishing a sabbatical. I forwarded him a few of my recent thoughts posted here, and here are his responses (which he has asked me to share with you until he can manage to get on the blog himself):

    “My view is that nowadays physical space has to mean the 4-dimensional space-time of relativity and in that context 3-dimensional space (a constant time slice) and perceptual space (a much more complicated slice) are intimately related. The problem is that both naive realists and critical realists think in 3-dimensional terms. Only 4-dimensional realism makes sense in the context of physical perception. One needs more than four dimensions in the context of non-physical perception - as well you know!”

    He agreed with my point that relativity theorists (including Einstein) made no distinction between naïve realism and critical realism in discussing the role of observation in relativity.

  38. Basically I think there is a fundamental error in shifting the locus of reality from perception to things we cannot perceive, cannot see with our “naked eyes,” to that which is beyond our senses. This “move” that critical realism espouses thereby confers on that which is not only unseen but perhaps unseeable a higher reality status (one might say this is a form of neoplatonism). Perhaps a more accurate characterization of the situation, though, is that we are *deceiving ourselves* into believing that we are “indirectly” perceiving the physical world with the aid of optical devices that magnify (microscopy, telescopy), imaging techniques, or various means of graphic and now computer visualization—all designed to render the invisible visible—voilà! The reverse of this procedure, i.e., making the visible invisible, is considered magic. “Primitive” peoples were once reported as thinking mirrors were magical—not to mention cameras that captured their souls (some American Indian tribes to this day are superstitious about photographing ceremonies.)

    But there is probably a fallacy in believing that we are thus somehow knowing, if not directly/indirectly perceiving, the physical world this way, by “extending” the senses, and it is likely that we are involved in a fundamental deception in the process. Meanwhile the whole quasi-magical notion of "making something visible" is never questioned and goes unanalyzed. Ergo my claim that there is a sort of legerdemain involved in what I am calling "third person naive realism," that magically eschews the subjective aspect of perception with the wave of the scientist’s wand, and as Schroedinger would say, removes the observer from the picture.

  39. I quite agree that it is not necessary to review Nicod's ideas any further: his main point—that topology is applicable to visual space—having been, I suggest, taken.
    I agree with Bob's remarks on the relation between phenomenal space and physical space. Here is a sketch of how Bob's theory of how intrinsic geometric properties of 2D visual space control distortions (or elimination of distortions) in the visual image (e.g. in peripheral vision) could be put in terms of Plato's cave. The prisoners chained to their stakes see only the shadows of the objects (in "physical space") cast by the light of the fire behind them on the wall of the cave in front of them (in "phenomenal space"). Now distortions in the shapes of these shadows could be induced (a) by suitable lenses in the beam of light behind the prisoners or (b) by changing the shape of the wall on which the shadows are thrown. The model for Bob's theory would then be (b). As Edmond Wright said (roughly) "We do not not see the shadows on the wall: we see the wall shadowed."

  40. Hear, hear, John! Since you share with theoretical physicist Saul-Paul Sirag an appreciation of Plato's analogy of the Cave, and its relevance to the problem of spatial dimensionality, I am glad you mentioned it. This just goes to show how old some of the questions we are discussing really are.

    In reading an abstract of the research conducted by Alby Richard and his associates at McGill on "The geometry of perisaccadic visual perception," I am in doubt as to what the abstract means by "surroundings." Are they referring to the visual world or physical world? It is not clear http://www.jneurosci.org/cgi/content/short/29/32/10160

  41. The question of how much of the apparent and measured geometry of visual space (VS) can be explained on the basis of projective geometry seems never to have been thoroughly explored, and whether some its curvature is (1) apparent or (2) an artifact of the method of measurement (my claim). Projection by definition involves a mapping down of dimensions, in this case (presumably) from a 3-space to a (curved?) 2-D surface, or even a series of such surfaces, because it is difficult to know how to describe the different depth planes seen in Random Dot Stereograms, for example. What is it that separates them along the (imaginary visual) z axis? They just appear to be at different depths, with *nothing* between them, thus perhaps another "primitive" element founding our notion of VS possessing spatial volume--as well as the idea of there being "empty" space.

  42. Rather than commenting here on Bob's paper, I shall do so on the comment feature attached to his paper.

  43. Re use of "surroundings" by Alby Richard et al., I imagine they mean the physical world, being explored by ordinary people in ordinary circumstances—but, owing to the general confusion in visual science between the 'visual field/world' and the 'stimulus field/physical word' that I have referred to earlier, the same phraseology could have been used to describe what the subjects in introspective psychological experiments do when they are observing their visual worlds under instructions by the psychologists.

  44. Perhaps you are right, John, but in my own experience I find that even the most enlightened vision scientists perpetually lapse into naive realism, and call both the stimulus field and visual world the "physical world." I dare say we all are guilty of that, too, which is perhaps only natural, because as Mach pointed out a century and more ago in his "Analysis of Sensations," that is just what scientists habitually do, i.e., the same configuration of sensations is called sensory by the psychologist in the context of his research, and physical by the physicist in his!

  45. In an era when the wisest of the wise (see e.g. Stephen Hawking in his new book "The Grand Design") claim that science has solved all problems in cosmology and physics, it does not seem proper for "the most enlightened vision scientists" to admit happily to such a lapse as Bill mentions, and then to continue as though, by doing this, they had not destroyed the very foundations of their own epistemology!

  46. Returning to Nicod for a moment, it would be interesting to see which of his concepts (e.g. interiority) correspond to existing notions of topological properties, in keeping with my own contention that both geometric and topological properties are derived from visual experience: First and foremost, they are *seen.*

  47. If you say that visual experiences are "seen", as a technical word, one is going to run into problems. Certainly common usage is that both external physical objects and hallucinations are "seen"—yet is it not better to say technically in psychology that objects are "seen" (but not experienced) and that all [visual in this case] sensations (including hallucinations) are "[visually] experienced" but not "seen"? "Seeing" involves the entire causal chain of visual perception: "experiencing" involves only the fin
    nal step.