What is Philosophy of Perception?
Why should there be such a thing as philosophy of perception? Why can’t psychologists and brain scientists say all there is to say about perception, without ‘help’ from philosophers? Some (like Dancy) answer this by saying that they deal with the most general questions in any given area. The idea is that philosophers are engaged in constructing high-level empirical theories which will accommodate and integrate what the latest scientists tell us about perception, whilst hopefully doing the least damage to our ordinary ideas about perception. This very popular way of approaching the philosophy of perception sees it as a kind of developing argument which begins with commonsense, and uses science and philosophical reasoning to critique it. The aim is to show how philosophical theories of perception grow out of commonsense by a process of sustained logical argument, in response to the evidence provided by hard empirical facts. I shall call this the theoretical conception of this branch of philosophy. (This is how the British Empiricists (Locke, Berkeley, Hume, Russell, and A.J.Ayer) felt about their own philosophical project, keen as they were on the idea that commonsense was not to be held up as the object of philosophical ridicule).
Common Sense and Naive Realism
If philosophy of perception begins by trying to identify commonsense ideas about perception that are the common property of all, or at least most, lay-people, where should we look for these commonsense ideas? Philosophers attached to the theoretical conception try to identify the informed commonsense which many of us are now party to and which does contain, in diluted, garbled and sensationalised form, traces of the scientific speculations of the recent past. The idea here is that our commonsense perceptual beliefs, which change relatively rapidly, as a result of the assimilation of scientific information, form or indicate in outline, a kind of theory, a commonsense theory of perception. And they think that we can set out this commonsense theory of perception, usually called ‘naive realism’ or ‘direct realism’, somewhat as follows:
One might think that this collection of statements is obviously or evidently true, and furthermore that one just couldn’t help believing it: haven’t these beliefs been shared, as John Hospers says, “by virtually all human beings” (ibid.)?
1. There exists a world of material objects.
2. Statements about these objects can be known to be true through sense-experience.
3. These objects exist not only when they are being perceived but also when they are not perceived. The objects of perception are largely, we might want to say, perception-independent.
4. These objects are also able to retain properties of the types we perceive them as having, even when they are not being perceived. Their properties are perception-independent.
5. By means of our senses, we perceive the world directly, and pretty much as it is. In the main, our claims to have knowledge of it are justified.
All this is familiar to us perceivers in virtue of having had, or heard about, certain experiences. We all know that perception can go wrong in these ways, and we even know how to correct for it in some cases. The naive 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. But philosophers have reacted to these facts in a more corrosive fashion.
A. 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’).
B. 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 Muller-Lyer illusion). (Call these phenomena ‘perceptual illusions’).
C. 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’).
Introducing Sense-Data: The Argument From Illusion
From considerations A, B and C comes a challenge to the Naive Realist theory. It’s called the ‘argument from illusion’, and it occurs prominently in the work of twentieth-century empiricists like A.J.Ayer and H.H.Price. Its purpose is to make us see the necessity of admitting the existence and ubiquity of sense-data. It goes like this:
The argument from illusion has convinced many philosophers that naive or direct realism is a false theory. It does nothing, of course, to defeat proposition 1 (There exists a world of material objects), nor proposition 2 (Statements about these objects can be known to be true through sense-experience) nor even proposition 3 (These objects exist not only when they are being perceived but also when they are not perceived), but it does suggest that proposition 4 (the perception-independence of the properties belonging to these objects) and the first part of proposition 5, that we perceive the world directly, and pretty much as it is, misrepresent the facts. For the argument from illusion, if successful, shows that we never directly see or otherwise directly perceive material objects, but only sense-data, that the world of material objects is known to us only via the intervention of a layer of mental proxies. This conclusion would certainly tell against any form of ‘naive realism’ or ‘direct realism’, as well as against common sense.
Objects appear differently to different observers, or differently to the same observer under different conditions. The way they appear is causally dependent upon environmental factors such as the presence of light, the position of the observer and the state of the observer’s nervous system. The fact that appearances vary in these ways suggests to us that sometimes people do not perceive things as they really are. But in every case where an object seems to be perceived there is something which is directly perceived, something which cannot appear otherwise than as it is. What’s more, there is a qualitative similarity between cases in which the object appears in its true colours and cases where it does not. There is nothing in the nature of the experience itself to mark off the one sort of case from the others.
But, it is now argued, in the cases where what is being directly perceived does not faithfully represent the object itself, what is being directly perceived is not a material object. It is a representation of, or an appearance of, or (as the classical Empiricists said) an idea of the object. If we take seriously the notion that these cases are sufficiently like the case of veridical perception, we may conclude that the material object involved is not directly perceived even in the veridical case. What is directly perceived is only ever an idea, a mental representation of the object. Nowadays these items are called ‘sense data’ (singular: sense-datum). Sense-data are perceived directly; material objects, on the other hand, if they can be said to be perceived, are perceived only indirectly.
Stages in the Argument
The first step in demolishing the argument from illusion is to set out the stages in the argument itself. In A.J.Ayer’s most famous presentation (The Foundations of Empirical Knowledge, (London: Macmillan, 1940)), adherents of the argument from illusion start by asking ‘What kind of thing are we directly aware of in perception?’. They suggest that the ordinary person’s answer to this question would be that we are directly aware of material things. The argument from illusion itself, Ayer says, “is based on the fact that material things may present different appearances to different observers or to the same observer in different conditions, and that the character of these appearances is to some extent causally determined by the state of the conditions and the observer” (p.3). From a consideration of examples like the straight stick half-immersed in water, it is suggested that “at least one of the visual appearances of the stick is delusive” (ibid., p.4). But even in the case where what we see is not a real quality of a material thing, it is argued, we are still seeing something. This then gets called a ‘sense-datum’, and is supposed to be that which we are directly aware of in perception. In more radical cases of non-standard perception, such as seeing mirages, it is said, no material thing is being perceived at all, but one’s experience “is not an experience of nothing, it has a definite content” (ibid., p.4). But if there’s no such material thing as the oasis I think I’m seeing, what is it that I am seeing? The answer: a sense-datum. So far this argument, if successful, establishes only that there are cases where the character of our perceptions makes it necessary to admit that what we are directly experiencing is not a material thing, but a sense-datum. How do these philosophers get to the conclusion that what we immediately experience is always a sense-datum, and never a material thing? They do so via three lines of argument.
The first, the argument from indistinguishability, says that there is no intrinsic difference in kind between those perceptions that are ‘veridical’ in their presentation of material things, and those that are ‘delusive’ (Price, p.31, Ayer, pp.5-6). The experiences involved are, allegedly, ‘qualitatively the same’. However, if, when our perceptions are delusive, we were always perceiving something of a different kind from what we perceived when they were veridical, we should expect our experience to be qualitatively different in the two cases. “We should expect to be able to tell from the intrinsic character of a perception whether it was a perception of a sense-datum or of a material thing” (Ayer, p.6). But in fact we can’t tell. So we ought to conclude that what is being perceived in the two cases are the same kind of things.
The second argument, the continuity argument, says that veridical and delusive perceptions may form a continuous series, both with respect to their qualities, and with respect to the conditions in which they are obtained (Price, p.32, Ayer, p.8). Ayer uses the example of gradually approaching an object from a distance. The idea is that the difference in quality between a veridical perception and its immediate predecessor (or successor) will be of the same order as the difference between any two successive delusive perceptions. But this isn’t what we should expect if the veridical perception were a perception of an object of a different sort. Since veridical and delusive perceptions shade imperceptibly into one another, the objects perceived in each case are generically the same. If so, and if we acknowledge that delusive perceptions are perceptions of sense-data, we must conclude that what we directly experience is always a sense-datum and never a material thing.
The third argument, from causal dependence, springs from the fact that our perceptions are somewhat dependent on external conditions and on our own physiological and psychological states. “[T]he relation between my perception and the accompanying conditions is such that, while they are not causally dependent on it, it is causally dependent on them” (Ayer, p.10). But it is characteristic of material things that their existence and their essential properties are independent of any particular observer. They are supposed to continue the same, whether they are observed by one person or another, or not observed at all. But this is not true of the objects of immediate experience. So the objects of immediate experience are not material things.
We may still be allowed, by the argument from illusion, to have ‘indirect’ knowledge of material things, but this must be obtained through the medium of sense-data, since these are the only things we are ever directly aware of in perception.