To: Christodoulos P.
From: Geoffrey Klempner
Subject: Criticism of the sense datum theory of perception
Date: 26 July 2007 15:47

Dear Christos,

Thank you for your email of 4 July, with your first essay for the Metaphysics program, in response to the question, 'What are the attractions of the idea of a ‘sense datum’? Can the theory of sense data be defended against criticisms based on the reality principle?'

Please accept my apologies for unintentionally going over my two week response deadline.

This is a carefully written and well researched response to the question which gives a clear and persuasive explanation of the attractions of the sense datum theory. However, you have less to say about the second part of the question, namely, whether the sense datum theory can be defended against criticisms based on the reality principle.

To sum up your argument: The argument from illusion states that the very same subjective experience can be interpreted as veridical experience or as hallucination or illusion. In that case, the immediate object of the experience must be the same in both cases, i.e. a sense datum.

Furthermore, if we look at the scientific account of the process of perception, we see that there is a causal chain leading from the object perceived to a state of the subject's brain. But the very same brain state can be produced by different causal chains, as when we compare normal perception with the Matrix scenario.

Finally, science tells us that we do not, in fact, perceive objects 'the way they are'. As you put it, 'the properties that science attributes to the object, properties such as weight, electromagnetic forces and fields, chemical substances and more', leading to the conclusion that the 'objects' which are immediately given to perception - defined in terms of colour, shape etc. - are merely representations of objects existing outside us which science describes.

First of all, it is important to get clear about the difference between attacking the argument for a particular theory, and attacking the theory itself.

As it happens, I do not accept these arguments for sense data because I think there is another explanation available in each of these three cases which does not imply the existence of sense data. However, that would not be a 'criticism of the sense datum theory based on the reality principle'. To give an analogy, if I attacked your reasons for believing in God (suppose you believe in God) that does not amount to a proof that God does not or cannot exist, only that your reasons are not good reasons.

So how does the reality principle apply to sense data?

Let's say that I claim to have a red, round sense datum. According to the reality principle, if this is a judgement (as it purports to be) then it is possible for me to be mistaken in making this judgement. But according to the sense datum theory, I cannot be wrong in my identifying the sense datum in question as red and round.

This is a clear case of inconsistency, which requires that we either reject sense data (as described) or reject (or amend) the reality principle.

You do in fact concede that 'sense data are the objects that we are aware of before arriving at any belief or making any judgements'. In other words, in order for the red, round sense datum to exist, in order for it to be 'given' to me as a sense datum, it is not necessary that I make any judgement about it. The problem with this is that I have just called it 'red' and 'round'. But that is a judgement. I am identifying an object and applying the predicates 'red' and 'round' to it.

A helpful analogy in thinking about judgement is that of an archer shooting arrows at a target. When you shoot the arrow, you aim to hit the target, and if your aim is bad then you miss. Making 'judgements' about one's own sense data is like shooting an arrow (in any direction) then painting a target around wherever the arrow happens to fall.

However, even if this criticism is accepted, it does not completely get rid of the sense datum idea. It is still tempting to say (and this is perhaps what you want to say) that the 'something' that is given cannot be described by any predicates or concepts 'just as it is'. Nevertheless, it is still something and not nothing.

I believe that this is what Kant wanted to say about what he termed 'intuition' or 'anschauung'. Whatever concepts we apply must, according to his 'Refutation of Idealism' be concepts that identify objects 'outside' us, in a spatio-temporal framework, rather than objects that exist subjectively 'in' us. There are not two objects, the 'tomato' in the world and the tomato-shaped and coloured sense datum in my mind. There is only the tomato on the plate, which is given to me in intuition and to which I apply the objective (not subjective) concepts 'red', 'round' etc.

Was Kant right? I argue in the program that this leads Kant inevitably to the distinction between the 'phenomenal world' constructed by applying concepts to intuition, and the 'noumenal world' about which we can have no knowledge. This is the essence of the theory that Kant calls 'transcendental idealism'.

However, there is another possible way in which one can 'save' these 'subjective objects' which does not require transcendental idealism. I don't go into this in the program, but if you are interested in pursuing this further you can look up my article, 'Truth and subjective knowledge' at http://klempner.freeshell.org/articles/shap.html

All the best,

Geoffrey