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Locke on primary and secondary qualities


To: Pearl K.
From: Geoffrey Klempner
Subject: Locke on primary and secondary qualities
Date: 2 May 2007 11:24

Dear Sachiko,

Thank you for your email of 1 May, with your University of London essay in response to the question, 'What grounds did Locke give for drawing a distinction between primary and secondary qualities? Do they provide an adequate basis for doing so?'

On the whole, this is a good, clear account of Locke's theory. I have an objection, however, to one of the criticisms that you make of Locke's theory. Also, there is an important criticism which you have not discussed.

Locke believed in the corpuscular theory, and uses this to explain the distinction between primary and secondary qualities. However, he makes clear in Book I that he is not in the business of defending any particular physical theory. There is no philosophical 'proof' of the corpuscular hypothesis. It is just a hypothesis, nothing more.

So, whatever arguments Locke puts forward for the distinction between primary and secondary qualities must be such as to stand up to scrutiny even if we are doubtful about the corpuscular theory.

On Locke's view, all that the philosopher, investigating the concept of primary and secondary qualities can say is that the primary properties of an object - solidity, extension, figure, motion and number - must account in some way for the occurrence of secondary qualities, and this is a matter about which we are free to put forward hypotheses. But ultimately, this is something which is beyond our knowledge.

At one point in his Essay, Locke considers the possibility that angels might be able to 'see' the corpuscles which constitute the structure of matter. Today, we have electronic microscopes. However, a point which Locke perhaps did not appreciate sufficiently is the possibility of positing unobservable entities in the spirit of 'hypothetico-deductive explanation'. It is not necessary to be able to 'see' the smallest constituents of matter in order to have well founded reasons for believing that they exist, namely, that they provide the 'best explanation' of observable phenomena.

However, this is a digression. Locke's arguments for the primary/ secondary quality distinction do not in fact depend on the corpuscular hypothesis. That red is a secondary quality, for example, is shown by the fact that nothing happens to a rose when you turn off the light. It is still physically the same object. But it's redness is gone. That shows that the property of redness is only the capacity of the rose to affect subjects who possess suitable sensory organs, i.e. a secondary quality.

Pain is special case, however. You give Locke's argument that pain is not a property of steel. However, there is a clear difference between the case of pain and red. The rose IS red. Redness is one of its secondary qualities. But we would not say that the steel blade 'is pain'. Why not? What is the difference?

The most well-known criticism of Locke's primary/ secondary quality distinction is the one made by Berkeley. According to Berkeley, the reasons that Locke gives for regarding, e.g. 'red' as a secondary quality equally show that what we term 'primary' qualities are also merely secondary, in that they depend on the capacity of an object to affect a subject. We have in fact no idea of primary qualities apart from secondary: any extension must be some colour, even if only black or grey.

Berkeley's criticism has to be understood against the background of a theory which considers the possibility that there is no such thing as 'matter', that all that exists are perceptions. As another way of putting your 'veil of perception' argument, we could say that Locke is assuming, without warrant, that material substance exists, whereas all we are given are perceptions.

In reply one might argue that there must be SOMETHING that causes these perceptions, in other words something that plays the part of matter and its primary qualities. For Kant, this 'something' is 'noumena' or 'things in themselves', which exist outside of space and time. For Berkeley, it is the infinite mind of God.

In both cases, however, one feels that the point of the primary/ secondary quality distinction has been lost. OK, it turns out from a metaphysical view that 'matter' is not ultimately real. Nevertheless, surely there is a point in distinguishing primary and secondary qualities along the lines the Locke distinguishes them, even if it turns out that the distinction is only relative from the point of view of metaphysics.

You allude to recent developments in physics, such as quantum mechanics. 'Matter' and 'space' are not what we thought they were. However, Locke can reply that so long as we accept that the physical world is ultimately real (contra Berkeley and Kant) then there will be some physical properties, WHATEVER THEY MAY BE, which ultimately account for the fact that we have perceptions of secondary qualities. And these, physics has discovered, are the real 'primary qualities'.

All the best,