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Locke's distinction between primary and secondary qualities


To: Scott B.
From: Geoffrey Klempner
Subject: Locke's distinction between primary and secondary qualities
Date: 28th April 2009 11:15

Dear Scott,

Thank you for your email of 21 April, with your essay in response to the University of London Modern Philosophy question, 'Why did Locke think it important to distinguish between primary and secondary qualities?'

Although the question doesn't state this, implicit in the question is the defensibility or otherwise of the primary/ secondary quality distinction as such. While you give a reasonably clear account of the arguments which Locke gives for making this distinction, what you don't state is how well these arguments stand up if we leave aside Locke's theory of ideas and qualities, and the problems which it gives rise to such as the 'veil of perception' and scepticism about an external world, as well as issues about the coherence of the corpuscular theory as Locke presents it.

Your account of Berkeley's criticisms of Locke further raises the suspicion that you are unnecessarily restricting yourself to the primary/ secondary quality distinction as it appears in Locke's theory, rather than examining the distinction as such on its own terms, and the arguments which Locke gives for the distinction, whose validity can be assessed independently of Locke's theory.

I hope that the distinction I am trying to make is reasonably clear. Let's start with Locke's theory.

You state that 'Locke possibly thinks that minute particles do not have secondary qualities.' This seems to follow, if we think of secondary qualities such as colour, taste or smell arising from a complex interaction between the corpuscular structure of an object and the corpuscular structure of our sense organs. A single corpuscle can only interact with another single corpuscle, according to Newton's laws of motion. In other words, on the level of corpuscles there cannot be such a thing as 'perception'. Locke at one point (I've searched for this passage in vain) imagines what it would be like if we had the senses of 'angels'. It seems that if you were small enough and could get up to a corpuscle, there is no mechanism whereby you could 'see' it. Perhaps you could grab hold, and give it a push. In which case, the single available mode of perception is proprioceptive feedback. On the other hand if 'angelic perception' does not depend on physical laws, then corpuscles can be any colour you like.

Another point to stress is that although Locke uses the variability of sensations to a perceiver (warm water feeling cold to one hand and hot to another) as a way of getting leverage on the idea of secondary qualities, he does not want to say that 'red' is a different 'colour' to every person who perceives it. Your example of the red coat which you like but your wife does not is not an example of two persons seeing different 'reds' but rather two persons having different likes/ dislikes regarding a particular quality of red.

As a matter of fact, there is wide agreement amongst human beings regarding what colours, smells, tastes things have. If I find that a particular brand of cheap supermarket cola 'tastes like chicken', I expect others to be able to make the same judgement. I don't mean that for some idiosyncratic reason it tastes like chicken to me, but rather I assume that there must be something in this particular brand of cola which produces the same taste effect as chicken, or, rather, more likely artificial chicken flavouring.

The uniformity of human colour judgements is very striking. There are standard tests to tell whether individuals are 'colour blind' and the variations of colour blindness.

Consider the angels again. The 'power' to produce sensations of colour has, Locke seems to have believed, a physical explanation in the corpuscular theory. However, this hypothesis is not strictly part of the primary/ secondary quality distinction as such. You can believe if you like that God directly intervenes in the process and gives us the appropriate sensations for any given object. That a tomato has the power to produce the sensation of red could be a matter of brute fact so far as we are concerned. All we know is that human beings agree on labelling the tomato (and similar objects) as 'red'.

More controversial, however, is Locke's belief that the primary qualities of objects are the ones which we identify as 'solidity, shape, extension, motion, number'. From the perspective of modern physics, we no longer live in a universe where apparently solid things are made of much smaller solid things. When you go all the way down, there's nothing 'solid' at all, indeed it is extremely hard to make any intelligible statement about what things are 'really like'. Should we then abandon the primary/ secondary quality distinction, or at least draw it differently? On the basis of modern physical theory, ought we to say that Locke was wrong that solidity is a primary quality, and that it is in fact a secondary quality?

Locke has a response to this, which is to insist that regardless of the truth or falsity of the corpuscular theory, it remains the case that the square shape of the table is part of the explanation of how it comes about that we see the table as being square (OR oblong). The explanation of perception does not assume any particular physical theory. Whereas primary qualities are correctly attributed to external objects, regardless of their ultimate constitution, secondary qualities are defined in terms of human agreement and disagreement in perception, regardless of one's theory of how perception arises.

All the best,