To: Richard E.
From: Geoffrey Klempner
Subject: Sense data and metaphysics of the reality principle
Date: 12 October 2005 09:58

Dear Richard,

Thank you for your email of 30 September, with your first essay for the Metaphysics program, in response to the question, 'Can the theory of sense-data be defended against criticism based on the reality principle?'

This is a good, clear answer to the question.

You start off by outlining the 'attractions' of the sense datum theory, focusing on the argument from illusion. As you correctly point out, the argument from illusion can itself be questioned, as Austin does (in his book Sense and Sensibilia). You write, 'We could say, as did the philosopher J.L. Austin, that there is only one stick which appears to be bent, but really is straight.'

A reader who was not familiar with Austin's work would find this statement rather puzzling, however. The whole point of the argument from illusion is that if the straight stick which I am looking at appears bent, then there must, in addition to the straight stick, be some 'object' which really is bent, namely the bent appearance or sense datum. How does one stop that move? Something extra needs to be said here to justify Austin's stance. But what, exactly?

Austin's method was to attend closely to ordinary usage. One of the things one notices when one does this, is that seeing is an action with success conditions. To grasp the nature of these success conditions, it is not necessary to invent a second sort of seeing, 'infallible seeing' which has no success conditions because it cannot ever fail. In other words, the argument from illusion is a simple non-sequitur. To fully appreciate this, however, one has to be prepared to do the 'hard work' of actually looking at real examples, as Austin does.

You then go on to consider possible ways in which the argument from the reality principle might be countered. One simple way to resist the argument would be to allow that judgements about sense data can be false. But what would that mean? The problem here is not just, as you argue, that the sense datum theory would lose the thing that made it attractive in the first place. To attack the motivation for a theory is not equivalent to a refutation of that theory. If the sense datum theory is useless and unattractive, it could still be true.

What one needs to do, in addition, is show that the very nature of 'sense data' is such that any attempt to describe a scenario in which judgements about them are false turns them into something different from what they were to begin with. They cease to be sense data and become just other objects in a 'recalcitrant world' concerning which it is possible to make false judgements.

Another line you consider is for the sense datum theorist to give up any pretence of constructing an external world on the basis of sense data, and admit the ontological primacy of spatio-temporal particulars. (This is how I interpret your statement, 'Another way to counter the above critique is to pose that the theory of sense data doesn't make judgements about objects *an sich*.')

This would be to give up the 'metaphysical' attraction of sense data, but there remain other powerful attractions. The main motivation for such a view is resistance to materialist theories of the mind. Sense data in this role are most frequently described as 'qualia'. Materialism, it is argued, cannot be true because qualia are real and cannot be 'reduced' (or statements about them cannot be reduced) to physical objects or events.

Unfortunately, this concession (if that is what it is intended to be) goes no way towards countering criticisms based on the reality principle.

Opponents of materialism need their 'sense data' or 'qualia' to be objects, things that exist, part of the furniture of the universe. If they are not part of the furniture of the universe, then they pose no threat to materialism. On the other hand, if they are part of the furniture of the universe, then it must be possible to make false judgements about them. If it is possible to make false judgements about them, then qualia lose the essential attribute which makes them incapable of being accounted for in materialist terms. - There is no way out of this tight circle except by rejecting the reality principle.

All the best,

Geoffrey