… and does not admit opposition of mathematical to physical certainty—here following Berkeley.
227. It has been shown [1] that Locke’s opposition of mathematical to physical certainty, with his ascription to the former of instructive generality on the ground of its bare ideality—the ‘ideal’ in this regard being opposed to what is found in sensation—strikes at the very root of his system. It implies that thought can originate, and that what it originates is in some sort real—nay, as being nothing else than the ‘primary qualities of matter,’ is the source of all other reality. Here was an alien element which ‘empiricism’ could not assimilate without changing its character. Carrying such a conception along with it, it was already charged with an influence which must ultimately work its complete transmutation by compelling, not the admission of an ideal world of guess and aspiration alongside of the empirical, but the recognition of the empirical as itself ideal The time for this transmutation, however, was not yet. Berkeley, in over-hasty zeal for God, had missed that only true way of finding God in the world which lies in the discovery that the world is Thought. Having taken fright at the ‘mathematical Atheism,’ which seemed to grow out of the current doctrines about primary qualities of matter, instead of applying Locke’s own admissions to show that these were intelligible and merely intelligible, he fancied that he had won the battle for Theism by making out that they were merely feelings or sequences of feelings. From him Hume got the text for all he had to say against the metaphysical mathematicians; but, for the reason that Hume applied it with no theological interest, its true import becomes more apparent with him than with Berkeley.
[1] See above, paragraphs 117 and 125.
His criticisms of the doctrine of primary qualities.
228. His account of mathematical truths, as contained in Part II. of the First Book of the ‘Treatise on Human Nature,’ cannot be fairly read except in connection with the chapters in Part IV. on ‘Scepticism with regard to the Senses,’ and on ‘the Modern Philosophy.’ The latter chapter is expressly a polemic against Locke’s doctrine of primary qualities, and its drift is to reverse the relations which Locke had asserted between them and sensations, making the primary qualities depend on sensations, instead of sensations on the primary qualities. In Locke himself we have found that two inconsistent views on the subject perpetually cross each other. [1] According to one, momentary sensation is the sole conveyance to us of reality; according to the other, the real is constituted by qualities of bodies which not only ‘are in them whether we perceive them or not,’ but which only complex ideas of relation can represent. The unconscious device which covered this inconsistency lay, we found, [2] in the conversion of the mere feeling of touch into the touch of a body, and thus into an experience of solidity. By this conversion, since solidity according to Locke’s account carries with it all the primary qualities, these too become data of sensation, while yet, by the retention of the opposition between them and ideas, the advantage is gained of apparently avoiding that identification of what is real with simple feeling, which science and common sense alike repel.
[1] See above, paragraph 99 and following.
[2] See above, paragraph 101.
It will not do to oppose bodies to our feeling when only feeling can give idea of body.
229. Hume makes a show of getting rid of this see-saw. Instead of assuming at once the reality of sensation on the strength of its relation to the primary qualities and the reality of these on the strength of their being given in tactual experience, he pronounces sensations alone the real, to which the primary qualities must be reduced, if they are not to disappear altogether. ‘If colours, sounds, tastes, and smells be merely perceptions, nothing we can conceive is possessed of a real, continued, and independent existence’. [1] That they are perceptions is of course undoubted. The question is, whether there is a real something beside and beyond them, contrast with which is implied in speaking of them as ‘merely perceptions.’ The supposed qualities of such a real are ‘motion, extension, and solidity’. [2] To modes of these the other primary qualities enumerated by Locke are reducible; and of these again motion and extension, according to Locke’s account no less than Hume’s own, presuppose solidity. What then do we assert of the real, in contrast with which we talk of perception, as mere perception, when we say that it is solid? ‘In order to form an idea of solidity we must conceive two bodies pressing on each other without any penetration. … Now, what idea do we form of these bodies? … To say that we conceive them as solid is to run on ad infinitum. To affirm that we paint them out to ourselves as extended, either resolves them all into a false idea or returns in a circle; extension must necessarily be conceived either as coloured, which is a false idea, [3] or as solid, which brings us back to the first question.’ Of solidity, then, the ultimate determination of the supposed real, there is ‘no idea to be formed’ apart from those perceptions to which, as independent of our senses, it is opposed. ‘After exclusion of colours, sounds, heat and cold from the rank of external existences, there remains nothing which can afford us a just and consistent idea of body.’
[1] p. 513 [Book I, part IV., sec. IV.]