To: Alan M.
From: Geoffrey Klempner
Subject: 'No-one really perceives a tree'
Date: 10 October 2001 12:58

Dear Alan,

Thank you for your essay for units 10-12 of the Possible World Machine, 'It is obvious that when you perceive a tree, what your eyes actually register is an upside-down image of a tree on the back of your retina. Therefore, no-one ever REALLY perceives the tree.'

How does the argument from the scientific account of the process of perception relate to the argument from illusion? You discuss the argument from illusion in the context where the scientific story is drafted in to explain how illusions can take place. This is an issue which very much interested Descartes in the 'Meditations'. For, having argued that there exists a God who is not a deceiver, he was then obliged to explain how it is that we do sometimes form false beliefs on the basis of our perceptions, and also suffer from illusory perceptions.

Descartes gives a basically mechanical story of causes and effects. Substituting nerve impulses for Descartes' 'motions of the animal spirits' results in a similar story of how the very same effect can arise from different causes. My subjective state of seeming to perceive a tree' might result, not from the typical cause of that effect, viz. my eyes taking in the information that there is a tree in front of me, but from some other cause. This is no fault in the way human beings are designed. We still have to use our best judgement, Descartes argues, in deciding whether in a particular case to accept the evidence of our senses.

However, the argument from illusion as such works quite independently of any scientific story of perception. We are invited to consider the consequences of the fact that the subjective character of experience is the same irrespective of its source. My subjective experience of seeing the tree will be the same, irrespective of whether or not there exists a world of material things in space. Things would appear to me just as they do now if my entire 'world' was an illusion produced by an evil demon, or so argues Descartes.

Similarly, the sense datum theory does not appeal to know facts about perception but takes as its starting point the idea that my subjective impression that there is a tree in front of me can exist in the absence of a tree.

Putting aside considerations relating to the argument from illusion, how convincing is the argument from the scientific account of perception? The target here is more subtle than the sense datum theory (where I would argue the private language argument has a major role to play in the critique). We are also not concerned with the problem of scepticism as such, the thought that for all I can prove, the whole of my 'life' might be a dream produced in me by an evil scientist.

You are right to point out that the upside down image on the retina is just one stage in the process. There is no special reason to pick on that stage rather than any other stage. But why then say that it is the tree that we perceive? Is it not more accurate to say that we infer the existence of a tree from information presented to our senses?

I think you go a considerable way towards diagnosing the fallacy here. Perception takes place in a context. You say, 'When a normal human adult sees a tree she brings to the encounter a wealth of prior knowledge and associations...it is language that enables her to differentiate the tree from its surrounding earth and sky'. This is part of the answer, though not the whole answer. It is also an essential part of the story that we ourselves are physical agents, of a particular size, with sense organs of a particular acuity. We are naturally designed to be able to detect certain kinds of fact by using our senses, without first having to make an inference from separate bits of information. This holds true, even though any claim to perceive an X can be undermined if certain facts which we had not considered are brought to our attention.

We generally see things as they are. This is in a sense guaranteed by the fact that we have given names to objects that we encounter in the world of our perception. This is the natural starting point in the process of making inferences and constructing theories to explain the things that we perceive. But we can also learn from theory to perceive things differently. Thus it is that, when we look up into the sky, we do not 'perceive' that the sun is the size of an orange. When we gaze out onto the horizon, we do not 'perceive' that the earth is flat. In these cases, we have learned to say, 'This is the way things look, but not the way things are.' By contast, when we look at a tree, in good lighting conditions with unimpaired eyesight, what we see is the same, in reality, as what is there.

All the best,

Geoffrey