"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 just 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:
The problem of the missing shade of blue arises because just two paragraphs later Hume seems to provide just such an idea. He says:
Responses to the problem
Some philosophers take Hume to task for presenting such a clear counter-example and then dismissing it as insignificant. Pritchard says:
Other philosophers take a more generous view of Hume's position. Jenkins says:
It is sometimes said that the problem is even more severe than Hume thinks. Hume claims that this instance is 'singular', but Alexander Broadie writes:
However, as Williams points out, Hume's own words imply that he was fully aware of this. Hume begins the relevant paragraph by talking about both sounds and colours. In addition, when first introducing the missing shade of blue he says, "except one particular shade of blue, for instance". The words "for instance" show that he could easily have chosen a different example. When he later says, "this instance is so singular, that it is scarcely worth our observing" he cannot be referring to this particular example, but rather to the type of exception that it represents.
When Hume says, "Let all the different shades of that colour, except that single one, be placed before him," he is assuming that colours are composed of a set of distinct independent hues, when in reality they form a continuum. However, Fogelin suggests, "Perhaps the reason that Hume does not see this is that he is thinking about the ideas of objects and not about objects themselves. In particular, he may hold that the notion of an indistinguishable difference between ideas make no sense. There is nothing more to an idea than that which can be discerned within it. If this is Hume's position, then the notion that two ideas can be different without being discernibly different would be a contradiction in terms."
Suggested solutions
A fully adequate solution to the problem will have the following features. It will:
Recognise that Hume believed the problem to be a genuine counter-example; Recognise that Hume included the example for a purpose; Provide an explanation that harmonizes well with other features of Hume's epistemology.
The problem has been tackled in various ways:
Mental mixing
Mental mixing is the solution proposed by Morris. The idea here is that just as paints are mixed to produce the range of colour swatches found in a hardware store, so it should be possible for colours to be mixed in the mind in some kind of analogous way. However, without further argument it is not obvious that we are endowed with any such ability and, if we were, it is not clear why it would be limited to the mixing of closely related impressions; yet, if this were not the case, then, contrary to what Morris says, it would open the floodgates to a range of philosophically suspect ideas.
Colours as complex ideas
Another way of dissolving the problem has been to suggest that colours might also be regarded as complex ideas. This is tempting since Hume has only spoken of "the faculty of compounding, transposing, augmenting, or diminishing the materials afforded us by the senses." That 'augmenting' and 'diminishing' do not apply only to physical size is clear from the way Hume suggests that our idea of God "arises from reflecting on the operations of our own mind, and augmenting, without limit, those qualities of goodness and wisdom." However, this fails our third criterion, for Hume clearly distinguishes between complex ideas and simple ideas in a way that excludes the possibility of colours being complex. In the treatise (Book1, Part1, Section1), Hume writes:
It does not undermine Hume's main concern
Hume's lack of concern might be explained by the fact that, although it contradicts the claim that all simple ideas are preceded by simple impressions, this is not a problem because it is not Hume's primary concern. This answer draws attention to what Hume says at the end of Section 2 of the Enquiries: