History of Western Philosophy - Bertrand Russell [376]
Hume's Treatise of Human Nature is divided into three books, dealing respectively with the understanding, the passions, and morals. What is important and novel in his doctrines is in the first book, to which I shall confine myself.
He begins with the distinction between 'impressions' and 'ideas'. These are two kinds of perceptions, of which impressions are those that have more force and violence. 'By ideas I mean the faint images of these in thinking and reasoning.' Ideas, at least when simple, are like impressions, but fainter. 'Every simple idea has a simple impression, which resembles it; and every simple impression a correspondent idea.' 'All our simple ideas in their first appearance are derived from simple impressions, which are correspondent to them, and which they exactly represent.' Complex ideas, on the other hand, need not resemble impressions. We can imagine a winged horse without having ever seen one, but the constituents of this complex idea are all derived from impressions. The proof that impressions come first is derived from experience; for example, a man born blind has no ideas of colours. Among ideas, those that retain a considerable degree of the vivacity of the original impressions belong to memory, the others to imagination.
There is a section (Book I, part i, sec. vii) 'Of Abstract Ideas', which opens with a paragraph of emphatic agreement with Berkeley's doctrine that 'all general ideas are nothing but particular ones, annexed to a certain term, which gives them a more extensive significance, and makes them recall upon occasion other individuals, which are similar to them.' He contends that, when we have an idea of a man, it has all the particularity that the impression of a man has. 'The mind cannot form any notion of quantity or quality without forming a precise notion of degrees of each.' 'Abstract ideas are in themselves individual, however they may become general in their representation.' This theory, which is a modern form of nominalism, has two defects, one logical, the other psychological. To begin with the logical objection: 'When we have found a resemblance among several objects,' Hume says, 'we apply the same name to all of them.' Every nominalist would agree. But in fact a common name, such as 'cat', is just as unreal as the universal CAT is. The nominalist solution of the problem of universals thus fails through being insufficiently drastic in the application of its own principles; it mistakenly applies these principles only to 'things', and not also to words.
The psychological objection is more serious, at least in connection with Hume. The whole theory of ideas as copies of impressions, as he sets it forth, suffers from ignoring vagueness. When, for example, I have seen a flower of a certain colour, and I afterwards call up an image of it, the image is lacking in precision, in this sense, that there are several closely similar shades of colour of which it might be an image, or 'idea', in Hume's terminology. It is not true that 'the mind cannot form any notion of quantity or quality without forming a precise notion of degrees of each.' Suppose you have seen a man whose height is six feet one inch. You retain an image of him, but it probably would fit a man half an inch taller or shorter. Vagueness is different from generality, but has some of the same characteristics. By not noticing it, Hume runs into unnecessary difficulties, for instance, as to the possibility of imagining a shade of colour you have never seen, which is intermediate between two closely similar shades that you have seen. If these two are sufficiently similar, any image you can form will be equally applicable to both of them and to the intermediate shade. When Hume says that ideas are derived from impressions which they exactly represent he goes beyond