The essence of imagination, I should say, is the absence of belief together with a novel combination of known elements. In memory, when it is correct, the combination of elements is not novel; and whether correct or not, there is belief. I say that in imagination there is “a novel combination of known elements”, because, if nothing is novel, we have a case of memory, while if the elements, or any of them, are novel, we have a case of perception. This last I say because I accept Hume’s principle that there is no “idea” without an antecedent “impression”. I do not mean that this is to be applied in a blind and pedantic manner, where abstract ideas are concerned. I should not maintain that no one can have an idea of liberty until he has seen the Statue of Liberty. The principle applies rather to the realm of images. I certainly do not think that, in an image, there can be any element which does not resemble some element in a previous perception, in the distinctive manner of images. Hume made himself an unnecessary difficulty in regard to the theory that images “copy” impressions. He asked the question: Suppose a man has seen all the different shades of colour that go to make up the spectrum, except just one shade. To put the thing in modern language, suppose he has never seen light of a certain small range of wave-lengths, but has seen light of all other wave-lengths. Will he be able to form an image of the shade he has never seen? Hume thinks he will, although this contradicts the principle. I should say that images are always more or less vague copies of impressions, so that an image might be regarded as a copy of any one of a number of different impressions of slightly different shades. In order to get a test case for Hume’s question, we should have to suppose that there was a broad band of the spectrum that the man had never seen--say the whole of the yellow. He would then, one may suppose, be able to form images which, owing to vagueness, might be applicable to orange-yellow, and others applicable to green-yellow, but none applicable to a yellow midway between orange and green. This is an example of an unreal puzzle manufactured by forgetting vagueness. It is analogous to the following profound problem: A man formerly hairy is now bald; he lost his hairs one by one; therefore there must have been just one hair that made the difference, so that while he had it he was not bald but when he lost it he was. Of course “baldness” is a vague conception; and so is “copying”, when we are speaking of the way in which images copy prototypes.