of all ideas. Hume accepts that ideas may be either the product of mere sensation or of the imagination working in conjunction with sensation. According to Hume, the creative faculty makes use of (at least) four mental operations that produce imaginings out of sense-impressions. These operations are compounding (or the addition of one idea onto another, such as a horn on a horse to create a unicorn); transposing (or the substitution of one part of a thing with the part from another, such as with the body of a man upon a horse to make a centaur); augmenting (as with the case of a giant, whose size has been augmented); and diminishing (as with Lilliputians, whose size has been diminished). (Hume 1974:317) In a later chapter, he also mentions the operations of mixing, separating, and dividing. (Hume 1974:340) However, Hume admits that there is one objection to his account: the problem of "The Missing Shade of Blue". In this thought-experiment, he asks us to imagine a man who has experienced every shade of blue except for one (see Fig. 1). He predicts that this man will be able to divine the color of this particular shade of blue, despite the fact that he has never experienced it. This seems to pose a serious problem for the empirical account, though Hume brushes it aside as an exceptional case by stating that one may experience a novel idea that itself is derived from combinations of previous impressions. (Hume 1974:319) 3. Of the association of ideas In this chapter, Hume discusses how thoughts tend to come in sequences, as in trains of thought. He explains that there are at least three kinds of associations between ideas: resemblance, contiguity in space-time, and cause-and-effect. He argues that there must be some universal principle that must account for the various sorts of connections that exist between ideas. However, he does not immediately show what this principle might be. (Hume 1974:320-321) 4. Sceptical doubts concerning the operations of the understanding (in two parts) In the first part, Hume discusses how the objects of inquiry are either "relations of ideas" or "matters of fact", which is roughly the distinction between analytic and synthetic propositions. The former, he tells the reader, are proved by demonstration, while the latter are given through experience. (Hume 1974:322) In explaining how matters of fact are entirely a product of experience, he dismisses the