(Extracted from Lehar's Gestalt Isomorphism paper.)
The perceived incredibility of both direct and indirect realism has led many over the centuries to propose that conscious experience is located neither in the physical brain, nor in the external world, but in some other separate space that bears no spatial relation to the physical space known to science. These theories fall somewhere between direct and indirect perception, because they claim that phenomenal experience is neither in the head, nor is it out in the world. The original formulation of this thesis was Cartesian dualism—the traditional religious or spiritual view that mind exists in a separate realm which is inaccessible to science. Our inability to detect spiritual entities is not due to any limitations of our detector technology, but to the fact that spiritual entities are impossible in principle to detect by physical means. Cartesian dualism is a minority position in contemporary philosophy, at least as a scientific theory of mind, and for very good reason. The chief objection to this kind of dualism is Occam's razor: it is more parsimonious to posit a single universe with one set of physical laws, rather than two radically dissimilar parallel universes composed of dissimilar substance and following dissimilar laws, making tenuous contact with each other nowhere else but within a living conscious brain. But if mind and matter come into causal contact, as they clearly do in both sensory and motor function, then surely they must be different parts of one and the same physical universe. But there is another, still more serious objection to Cartesian dualism than the issue of parsimony. Since the experiential, or spiritual component of the theory is in principle inaccessible to science, that portion of the theory can be neither confirmed nor refuted. This places the spiritual component of Cartesian dualism beyond the bounds of science, and firmly in the realm of religious belief.
A more sophisticated half-way epistemology is seen in the philosophy of critical realism (Sellars 1916, Russell 1921, Broad 1925, Drake et al. 1920). While the critical realists avoid religious explanations involving God or spirits, their concept of conscious experience nevertheless preserves some of the mystery of Cartesian dualism. Critical realists acknowledge that perception is not direct, but is mediated by an intermediate representational entity which they call sense-data. However critical realists insist that the sense-data are neither physical nor mental, but "particular existents of a peculiar kind; they are not physical, ... and there is no reason to suppose that they are either states of mind or existentially mind-dependent. In having spatial characteristics ... they resemble physical objects ... but in their privacy and their dependence on the body ... of the observer they are more like mental states." (Broad 1925, p. 181) As with the spirit world of the Cartesian view, sense data and the space in which they are observed are not just difficult to detect, but they are in principle beyond scientific scrutiny. There is some debate among critical realists over the ontology of conscious experience. In a book on critical realism by a consortium of authors (Drake et al. 1920) Lovejoy, Pratt, and Sellars claim that the sensa are completely "the character of the mental existent ... although its existence is not given", while Drake, Rogers, Santayana, and Strong agree that the data are characteristic of the apprehended object, although "the datum is, qua datum, a mere essence, an inputed but not necessarily actual existent. It may or may not have existence." (p. 20-21 footnote). So the critical realists solved the epistemological problem by defining a unique kind of existent that is experienced, but that does not, or may not actually exist. This is a peculiar inversion of the true epistemological situation, because in fact sense data, or the raw material of conscious experience, are the only thing which we can know with any real certainty to actually exist. All else, including the entire physical world known to science, is informed conjecture based on that experience.
A more modern reformulation of this muddled epistemology is seen in Davidson's anomalous monist thesis (Davidson 1970). Davidson suggests that the mental domain, on account of its essential anomalousness and normativity cannot be the object of serious scientific investigation, because the mental is on a wholly different plane from the physical. This argument sounds like the metaphysical dualism of Descartes that disconnects mind from brain entirely, except that Davidson qualifies his theory with the monistic proviso that every mental event is connected with specific physical events (in the brain), although there are no laws connecting mental kinds with physical kinds, and this presumably rescues the thesis from metaphysical dualism. Kim (1998) points out however that this is a negative thesis, for it tells us only how the mental is not related to the physical, it says nothing about how they are related. As such this is more an article of faith rather than a real theory of any sort, and in the context of the history of the epistemological debate this can be seen as a last desperate attempt to rescue na´ve realism from its own logical contradictions. This kind of physicalism has been appropriately dubbed `token physicalism', for it is indeed a token admission of the undeniable link between mind and the physical brain, without admitting to any of its very significant implications.
In order to rationalize this view of the mind-brain relation Davidson introduces the peculiar notion of supervenience, a one-way asymmetrical relation between mind and brain that makes mind dependent on the brain, but that forever closes the possibility of phenomenological observation of brain states. As in the case of Cartesian dualism, there are two key objections to this argument. In the first place the disconnection between the experiential mind and the physical brain is itself merely a hypothesis, whose truth remains to be demostrated. It is at least equally likely prima facie that the mind does not supervene on the brain, but that mind is identically equal to the functioning of the physical brain. In fact, this is by far the more parsimonious explanation, because it invokes a single explanans, the physical brain, to account for the properties of both mind and brain. After all, physical damage to the brain can result in profound changes in the mind—not just the information content of the mind, nor just observed behavior, but brain damage can produce profound changes in the experiential, or "what it is like" aspect of conscious experience. The simplest explanation therefore is that consciousness is a physical process taking place in the physical brain, which is why it is altered by physical changes to the physical brain.
But the problem of supervenience is more serious than just the argument of parsimony. For if the properties of mind were indeed disconnected from the properties of the physical brain, this would leave the mental domain completely disconnected from the world of reality known to science, what Feigl (1958) has called a "nomological dangler." For if the properties of mind are not determined by the properties of the physical brain, what is it that determines the properties of the mind? For example phenomenal color experience has been shown to be reducible to the three dimensions of hue, intensity, and saturation. Physical light is not restricted to these three dimensions; the spectrum of a typical sample of colored light contains a separate and distinct magnitude for every spectral frequency of the light, an essentially infinite-dimensional space that is immeasurably greater in information content than the three dimensions of phenomenal color experience. In answer to Koffka's (1935) classical question—"Why do things look as they do?" the answer is clearly not "Because they are what they are." That answer that is clearly false in the case of color perception, as well as in the case of visual illusions, not to mention dreams and hallucinations. We now know that the dimensionality of color experience relates directly to the physiology of color vision, i.e. it relates to the fact that there are three different cone types in the human retina, and to the opponent color process representation in the visual cortex. The dimensions of color experience therefore are not totally disconnected from the properties of the physical brain as suggested by Davidson, but in fact phenomenal color experience tells us something very specific about the properties of the representation of color in the physical brain. And the same argument holds for spatial vision, for there are a number of prominent distortions of phenomenal space that clearly indicate that phenomenal space is ontologically distinct from the physical space known to science, as will be discussed later.
Daniel Dennett (1991) promotes a similar half-way epistemology by drawing a distinction between the neural vehicles of mental representation, and the phenomenal contents of those vehicles. Dennett opens the epistemological crack by claiming that the phenomenal contents do not necessarily bear any similarity whatsoever to the neural vehicles by which they are encoded in the brain. This actually goes beyond Davidson's supervenience, because according to Davidson, mental events that are distinct phenomenally, must be distinct neurophysiologically also. This is tantamount to saying that the dimensions of conscious experience cannot be any greater than the dimensions of the corresponding neurophysiological state. Dennett effectively removes this limitation by suggesting that even the dimensionality of the phenomenal contents need not match that of the neural vehicles. And into that epistemological crack, Dennett slips the entire world of conscious experience like a magical disappearing act, where it is experienced, but does not actually exist. But by the very fact that conscious experience, as conceived by Dennett, is in principle undetectable by scientific means, this concept of consciousness becomes a religious rather than a scientific hypothesis, whose existence can be neither confirmed nor refuted by scientific means. In fact Dennett even suggests that there is actually no such thing as consciousness per se, and that belief in consciousness is akin to belief in some kind of mythical nonexistent deity (Dennett 1981). This argument of course is only intelligable from a na´ve realist perspective, by which the sense-data of conscious experience, so plainly manifest to one and all, are mis-identified as the external world itself, rather than as something going on in the physical brain.
Another modern theorist, Max Velmans (1990), revives an ancient notion of perception as something projecting out of the head into the world, as proposed by Empedocles and promoted by Malebranche. But Velmans refines this ancient notion with the critical realist proviso that nothing physical gets actually projected from the head, the only thing that is projected is conscious experience, a subjective quality that is undetectable externally by scientific means. But again, as with critical realism, the problem with this notion is that the sense data that are experienced to exist, do not exist in any true physical sense, and therefore the projected entity in Velman's theory is a spiritual entity to be believed in, (for those who are so inclined) rather than anything knowable by, or demonstrable to science. Velmans draws the analogy of a videotape recording, that carries the information of a dynamic pictorial scene expressed in a highly compressed and non-spatial representation, as patterns of magnetic fields on the tape. There is no resemblance or isomorphism between the magnetic tape and the images that it encodes, except for its information content. However the only reason that the videotape even represents a visual scene is because of the existence of video technology that is capable of reading that magnetic information from the tape and sweeping it out as a spatial image on a video monitor or television screen, where each pixel appears in its proper place in the image. If that equipment did not exist, then there would be no images as such on the videotape. But if video technology is to serve as an analogy for spatial representation in the brain, the key question is whether the brain encodes that pictorial information exclusively in the abstract compressed form like the magnetic patterns on the tape, or whether the brain reads those compressed signals and projects them as an actual spatial image somewhere in the brain like a television monitor, whenever we have a visuospatial experience. For if it is the former, then sense data are experienced, but do not actually exist as a scientific entity, so the spatial image we see is a complete illusion, which again, is an inversion of the true epistemology. If it is the latter, then that means that there are actual "pictures in the head", a notion that Velmans emphatically rejects.
In fact the only epistemology that is consistent with the modern materialistic world view is an identity theory (Russell 1927, Feigl 1958) whereby mind is identically equal to physical patterns of energy in the physical brain. To claim otherwise is to relegate the elaborate structure of conscious experience to a mystical state beyond the bounds of science. The dimensions of conscious experience, such as phenomenal color and phenomenal space, are a direct manifestation of certain physical states of our physical brain. The only correct answer to Koffka's question is that things appear as they do because that is the way the world is represented in the neurophysiological mechanism of our physical brain. The world of conscious experience therefore is in principle accessible to scientific scrutiny after all, both internally through introspection, and externally through neurophysiological recording. And introspection is as valid a method of investigation as is neurophysiology, just as in the case of color experience. Of course the mind can be expected to appear quite different from these two different perspectives, just as the data in a computer memory chip appears quite different when examined internally by data access, as opposed to externally by electrical probes. But the one quantity that is preserved across the mind/brain barrier is information content, and therefore that quantity can help identify the neurophysiological mechanism or principle in the brain whose dimensionality, or information content, matches the observed dimensions of conscious experience.