The cabbage view is incredible because it suggests that we can have experience of objects out in the world directly, beyond the sensory surface, as if bypassing the chain of sensory processing. For example if light from your screen is transduced by your retina into a neural signal which is transmitted from your eye to your brain, then the very first aspect of the computer screen that you can possibly experience is the information at the retinal surface, or the perceptual representation that it stimulates in your brain. The physical monitor itself lies beyond the sensory surface and therefore must be beyond your direct experience. But the perceptual experience of the page stubbornly appears out in the world itself instead of in your brain, in apparent violation of everything we know about the causal chain of vision. The difficulty with the concept of direct perception is most clearly seen when considering how an artificial vision system could be endowed with such external perception. Although a sensor may record an external quantity in an internal register or variable in a computer, from the internal perspective of the software running on that computer, only the internal value of that variable can be "seen", or can possibly influence the operation of that software. In exactly analogous manner the pattern of electrochemical activity that corresponds to our conscious experience can take a form that reflects the properties of external objects, but our consciousness is necessarily confined to the experience of those internal effigies of external objects, rather than of external objects themselves. Unless the principle of direct perception can be demonstrated in a simple artificial sensory system, this explanation remains as mysterious as the property of consciousness it is supposed to explain.
The indirect realist view is also incredible, for it suggests that the solid stable structure of the world that we perceive to surround us is merely a pattern of energy in the physical brain. In other words, the world that appears to be external to our head is actually inside our head. This could only mean that the head we have come to know as our own is not our true physical head, but is merely a miniature perceptual copy of our head inside a perceptual copy of the world, all of which is completely contained within our true physical skull. Stated from the internal phenomenal perspective, out beyond the farthest things you can perceive in all directions, i.e. above the dome of the sky and below the earth under your feet, or beyond the walls, floor, and ceiling of the room you perceive around you, beyond those perceived surfaces is the inner surface of your true physical skull encompassing all that you perceive, and beyond that skull is an unimaginably immense external world, of which the world you see around you is merely a miniature virtual reality replica. The external world and its phenomenal replica cannot be spatially superimposed, for one is inside your physical head, and the other is outside. Therefore the vivid spatial structure of this page that you perceive here in your hands is itself a pattern of activation within your physical brain, and the real paper of which it is a copy is out beyond your direct experience. Although this statement can only be true in a topological, rather than a strict topographical sense, this insight emphasizes the indisputable fact that no aspect of the external world can possibly appear in consciousness except by being represented explicitly in the brain. The existential vertigo occasioned by this concept of perception is so disorienting that only a handful of researchers have seriously entertained this notion or pursued its implications to its logical conclusion.
The key to this problem of fitting a spacious world into our brains is to notice that our experience is a 'view' of a spacious world. Things are separated by angles relative to an observation point. The separation of things by angles at a point means that we do not have a
sense of depth that operates in the same way as our sense of things being separated in horizontal and vertical directions. Our sense of depth is based upon cues rather than an actual experience of the space between things. As an example, the stars in a planetarium appear incredibly distant even though they are on the ceiling of a room and would appear just as distant if viewed through virtual reality goggles. Visual depth in particular is a set of inferences, not an actual experience of the space between things in a radial direction outward from the observation point. This means that the things that are the spacious world of experience could be as small as just a few cubic millimetres of brain tissue!
If there is anything to be learned from the long history of the epistemological debate, it is that the issue is by no means simple or trivial, and that whatever is ultimately determined to be the truth of epistemology, we can be sure that it will do considerable violence to our common-sense view of things. This however is nothing new in science, for many of the greatest discoveries of science seemed initially to be so incredible that it took decades or even centuries before they were generally accepted. But accepted they were, eventually, and the reason why they were accepted was not because they had become any less incredible. In science, irrefutable evidence triumphs over incredibility, and this is exactly what gives science the power to discover unexpected or incredible truth.
We should remember that the following considerations are also part of informed commonsense.
- What we perceive is often dependent on our organs of perception and their condition. If we had compound eyes, as flies do, we would receive information about the visual world in a completely different form. If we had jaundice, things would look yellow. If we had other sense organs altogether, like infra-red detectors or echo-location devices, things might appear to us in ways which we can’t even imagine. (Let’s call this ‘perceptual variability’).
- Even our current perceptual apparatus is obviously not infallible. We are all familiar with perceptual illusions of various sorts. A major sub-classification of such illusions relates to whether the sensory organs are malfunctioning (as in jaundice) or whether they habitually misrepresent objects to us even in full working order (e.g. the Müller-Lyer illusion).
- Sometimes these perceptual illusions extend to cases where we think we perceive things which in fact aren’t there at all (rather than just misperceiving the properties of things which are there to be perceived). This is a more radical case of perceptual error than simple illusion. (Call it ‘hallucination’ or ‘perceptual delusion’).
The basic claim is that in cases of illusion or hallucination, the object that is immediately experienced or given has qualities that no public physical object in that situation has and so must be distinct from any such object. And in cases of perceptual relativity, since objects with different qualities are experienced from each of the different perspectives or under each of the relevant conditions, at most one of these various immediately experienced or given objects could be the physical object itself; it is then further argued that since there is no apparent experiential basis for regarding one out of any such set of related perceptual experiences as the one in which the relevant physical object is itself immediately experienced, the most reasonable conclusion is that the immediately experienced or given object is always distinct from the physical object. (Or, significantly more weakly, that there is no way to identify which, if any, of the immediately experienced objects is the physical object itself, so that the evidential force of the experience is in this respect the same in all cases, and it is epistemologically as though physical objects were never given, whether or not that is in fact the case.)
The naïve realist theory of perception is not threatened by these facts as they stand, for they are accommodated by that theory by virtue its very vagueness (or ‘open-texture’). The theory just isn’t specific or detailed enough to be refuted by the (actually very rare) occurrence of these cases.
The cogency of this argument has been challenged in a number of different ways, of which the most important are the following. First, it has been questioned whether there is any reason to suppose that in cases of these kinds there must be some object present that actually has the experienced qualities, which would then seemingly have to be something like a sense-datum. Why couldn't it be that the perceiver is simply in a state of seeming to experience such an object without any object actually being present? (See the discussion below of the adverbial theory.) Second, it has been argued that in cases of illusion and perceptual relativity at least, there is after all an object present, namely the relevant physical object, which is simply misperceived, for the most part in readily explainable ways. Why, it is asked, is there any need to suppose that an additional object is also involved? Third, the last part of the perceptual relativity version of the argument has been challenged, both (i) by questioning whether it is really true that there is no experiential difference between veridical and non-veridical perception; and (ii) by arguing that even if sense-data are experienced in non-veridical cases and even if the difference between veridical and non-veridical cases is, as claimed, experientially indiscernible, there is still no reason to think that sense-data are the immediate objects of experience in veridical cases. Fourth, various puzzling questions have been raised about the nature of sense-data: Do they exist through time or are they momentary? Can they exist when not being perceived? Are they public or private? Can they be themselves misperceived? Do they exist in minds or are they extra-mental, even if not physical? On the basis of the intractability of these questions, it has been argued that the conclusion of the argument from illusion is clearly unacceptable or even ultimately unintelligible, even in the absence of a clear diagnosis of exactly where and how it goes wrong.
