Sensation and Perception

Put a coin on the table and look at it from the side. It is round and it looks elliptical.

Nothing about that sentence is controversial and almost every position in the philosophy of perception has to explain it. The explanations turn out to differ so sharply that a large part of the subject can be reconstructed from this one observation, and the first thing to come under pressure is the ordinary belief that when you look at a coin you see a coin.

The state under examination

Two features of the definition are doing work.

The first is the phrase "or seem to". A perceptual experience is picked out by its character rather than by its success, so a person taken in by a convincing fake and a person seeing the real thing can be having experiences of the same type. Whether they really are is the question the rest of this article circles.

The second is that experience is not belief. You can see the coin as elliptical while believing it is round, and the belief does not correct the look. This is easiest to check with a standard geometrical illusion: measure two lines, confirm they are the same length, and they continue to look different. Whatever perceptual experience is, it is not a matter of what you take to be the case, since it survives your taking the opposite.

The traditional division puts sensation at the bottom and perception above it, with sensation delivering unstructured material and perception organising it into a world of objects. Seeing a red patch is sensation; seeing a tomato is perception.

The division is useful and it should be held loosely, because a great deal of evidence suggests that no such stage exists on its own. Perception is organised from very early on, and what is delivered to consciousness is already structured into surfaces and objects rather than assembled from a mosaic afterward. Nobody has managed to point to a moment of pure sensation in an adult perceiver, and some philosophers hold there is no such thing.

Even so, the distinction marks a real difference in what a theory is trying to explain. A question about how a system detects a wavelength is a different question from how it comes to be seeing a tomato, and running them together produces confusion in both directions.

The trouble starts with ordinary cases

Return to the coin. It is round, and it looks elliptical, and now a premise gets introduced that looks harmless.

If something looks a certain way to you, then there is something you are aware of that is that way. In the literature this is the phenomenal principle, and its appeal is that it seems to be the only way to take appearances seriously. If nothing elliptical were involved anywhere, what would make it true that something looked elliptical rather than triangular?

Apply the principle to the coin and you get a result at once. Something elliptical is present to your awareness. The coin is not elliptical. So the thing you are aware of is not the coin.

The second half is where the argument becomes ambitious, and it is the half most people miss on first reading. Establishing something about illusions alone would be a minor result. The argument gets its reach from a further claim, that veridical and illusory experiences do not differ in kind, so that whatever account is given of one has to be given of the other.

That claim is not arbitrary. The experiences can be indistinguishable from the inside, and the ordinary case shades continuously into the illusory one as you tilt the coin. Drawing a line anywhere on that continuum and saying that awareness has a different object on either side of it looks unmotivated. If you are aware of something elliptical when the coin is tilted a lot, it is hard to say what changes when it is tilted a little.

The versions of this argument were developed carefully in the early twentieth century by G. E. Moore and Bertrand Russell, and it was set out in the form best known to later readers by A. J. Ayer in 1940. Russell's own route ran through variation rather than error: a table looks different in colour and shape from every position and in every light, no one of those appearances has a better claim than the others to be how the table is, and the table itself does not change as you walk around it.

The term was introduced into English philosophy by Moore in a paper read to the Aristotelian Society at the end of 1909, and Russell adopted it. The theory built on it holds that we are directly aware of sense data and only indirectly aware of physical objects, which we know about by inference or construction from the data. It was the dominant account of perception for several decades, and the intermediate tier examines it properly along with the objections that eventually sank it.

Three of those objections are worth previewing, since they show why the position is a costly one. Sense data do not fit anywhere in the physical world, being neither in the head nor out where the coin is, so a physicalist who accepts them owes an account of what they are. They also seem to have indeterminate properties in a way ordinary objects cannot: a speckled hen seen briefly presents an experience with speckles that has no definite number of them, and an object with no definite number of speckles is a strange item. And positing them puts a screen between the perceiver and the world, which raises the question of how anyone ever knew about the coin.

Hallucination is worse

Illusion involves a real object misrepresented. Hallucination involves no object at all, and it produces a parallel argument that does not need the phenomenal principle to get going.

In a hallucination there is no ordinary object being perceived, so whatever the subject is aware of, it is not one. If a hallucination can be indistinguishable from a veridical perception, and there is no evident reason it could not be, then the two experiences are of the same kind. And if they are of the same kind, the account given of the hallucination has to be given of the perception too.

This version is the sharper of the two, because it cannot be met by denying that anything really is elliptical. There is simply nothing there, and the subject is still undergoing something with a full perceptual character.

The step that has attracted the most scrutiny is the move from indistinguishability to sameness of kind, sometimes called the common kind assumption. Two things being impossible to tell apart from the inside does not obviously make them the same sort of thing, and a family of positions developed in the last few decades denies exactly this, holding that a genuine perception and a matching hallucination are fundamentally different states that happen to be introspectively indiscriminable. The intermediate tier takes that up.

Next

Everything in this article has relied on subjects being able to say how things seem to them. That reliance has been unexamined, and it is not obviously safe. The next article asks how good anyone's access to their own mind actually is.