The Missing Shade of Blue "The Missing Shade of Blue" is an example introduced by the Scottish philosopher David Hume to show that it is at least conceivable that the mind can generate an idea without first being exposed to the relevant sensory experience. It is regarded as a problem by philosophers because it appears to stand in direct contradiction to what Hume had previously written. ## The source of the problem In both A Treatise of Human Nature and An Enquiry Concerning Human Understanding, philosopher David Hume argues that all perceptions of the mind can be classed as either 'Impressions' or 'Ideas'. He further argues that: We shall always find, that every idea which we examine is copied from a similar impression. Those who would assert, that this position is not universally true nor without exception, have only one, and at that an easy method of refuting it; by producing that idea, which, in their opinion, is not derived from this source. The Missing Shade of Blue The problem of the missing shade of blue arises because just two paragraphs later Hume seems to provide just such an idea. He says: There is, however, one contradictory phenomenon, which may prove, that it