History of Western Philosophy - Bertrand Russell [348]
This, however, is a paradox, and Locke will have nothing to do with paradoxes. Accordingly, in another chapter, he sets forth a different theory, quite inconsistent with the earlier one. We have, he tells us, three kinds of knowledge of real existence. Our knowledge of our own existence is intuitive, our knowledge of God's existence is demonstrative, and our knowledge of things present to sense is sensitive (Book IV, chap. iii).
In the next chapter, he becomes more or less aware of the inconsistency. He suggests that someone might say: 'If knowledge consists in agreement of ideas, the enthusiast and the sober man are on a level.' He replies: 'Not so
where ideas agree with things.' He proceeds to argue that all simple ideas must agree with things, since 'the mind, as has been showed, can by no means make to itself' any simple ideas, these being all 'the product of things operating on the mind in a natural way'. And as regards complex ideas of substances, 'all our complex ideas of them must be such, and such only, as are made up of such simple ones as have been discovered to coexist in nature'. Again, we can have no knowledge except (1) by intuition, (2) by reason, examining the agreement or disagreement of two ideas, (3) 'by sensation, perceiving the existence of particular things' (Book IV, chap. iii, sec. 2). In all this, Locke assumes it known that certain mental occurrences, which he calls sensations, have causes outside themselves, and that these causes, at least to some extent and in certain respects, resemble the sensations which are their effects. But how, consistently with the principles of empiricism, is this to be known? We experience the sensations, but not their causes; our experience will be exactly the same if our sensations arise spontaneously. The belief that sensations have causes, and still more the belief that they resemble their causes, is one which, if maintained, must be maintained on grounds wholly independent of experience. The view that 'knowledge is the perception of the agreement or disagreement of two ideas' is the one that Locke is entitled to, and his escape from the paradoxes that it entails is effected by means of an inconsistency so gross that only his resolute adherence to common sense could have made him blind to it.
This difficulty has troubled empiricism down to the present day. Hume got rid of it by dropping the assumption that sensations have external causes, but even he retained this assumption whenever he forgot his own principles, which was very often. His fundamental maxim, 'no idea without an antecedent impression', which he takes over from Locke, is only plausible so long as we think of impressions as having outside causes, which the very word 'impression' irresistibly suggests. And at the moments when Hume achieves some degree of consistency he is wildly paradoxical.
No one has yet succeeded in inventing a philosophy at once credible and self-consistent. Locke aimed at credibility, and achieved it at the expense of consistency. Most of the great philosophers have done the opposite. A philosophy which is not self-consistent cannot be wholly true, but a philosophy which is self-consistent can very well be wholly false. The most fruitful philosophies have contained glaring inconsistencies, but for that very reason have been partially true. There is no reason to suppose that a self-consistent system contains more truth than one which, like Locke's, is obviously more or less wrong.
Locke's ethical doctrines are interesting, partly on their own account,