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.
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.
I believe it will readily be allowed, that the several distinct ideas of colour, which enter by the eye, or those of sound, which are conveyed by the ear, are really different from each other; though, at the same time, resembling.
Suppose, therefore, a person to have enjoyed his sight for thirty years, and to have become perfectly acquainted with colours of all kinds, except one particular shade of blue, for instance, which it never has been his fortune to meet with.
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.
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."
By bringing ideas into so clear a light we may reasonably hope to remove all dispute, which may arise, concerning their nature and reality.The fact that this contradicts Hume's explicitly stated purpose is explained by arguing that the Treatise has to be understood as a gradual unfolding of his views.
The problem is that Hume never makes this clear, and if this is the way it is meant to be read then, as Williams says, "The narrative character of the Treatise is...disguised...by the superficial resemblance of the former to Locke's Essay... Also, there is the fact that he drops the problem in the same way in the Enquiry, which arguably lacks the narrative character of the Treatise.
The proviso that they do not share anything in common is important because otherwise this feature might be separated off and this would show that the original idea was in fact complex.
If it is allowed that the notion of hue can arise through abstraction even though it cannot in any instance be separated from a given example, then it may be fairly argued that the ability to fill a gap in the colour space is quite a different matter from coming up with an isolated idea without any prior impression.
Suppose, therefore, a person to have enjoyed his sight for thirty years, and to have become perfectly acquainted with regular polygons of all kinds except the one having five sides... By whatever means the idea of the missing shade is to be created, there is still the question of why Hume takes such pains to present the example to his readers.
On the other hand, Nelson[10] suggests the intriguing possibility that far from being an oversight or an embarrassment to his wider project, the missing shade of blue example turns out to be crucial.
With this in mind it can be asked what status holds for the claim that "all our ideas or more feeble perceptions are copies of our impressions or more lively ones".
If this is a Relation of Ideas, then it does not necessarily say anything true about the world, and this will not suit Hume's purpose at all; if it is a Matter of Fact, then the contrary must be possible.
And that's why Hume can reasonably say that this contrary evidence can be disregarded or, as he puts it, that «it is scarcely worth our observing, and does not merit, that for it alone we should alter our general maxim».
[1] Against such interpretation is the fact that Hume himself in Section II calls the "missing shade of blue" as a «proof, that the simple ideas are not always, in every instance, derived from the correspondent impressions»,[1] where in Section VI he defines "proof" as not a demonstrative argument but as an argument from experience that «leaves no room for doubt or opposition».
