What point was Hume trying to make with the missing shade of blue in the Treatise of Human Nature?
Answer by Craig Skinner
The existence of the idea of a missing shade of blue contradicts Hume’s Copy Principle that simple ideas all derive from antecedent simple impressions. But he dismisses this ‘exception’ as unimportant. Why then does he mention it, and, as you say what point is he trying to make.
He deals with the matter in exactly the same way in both the Treatise and, eight years later, in the first Enquiry, so that it is no throwaway line or momentary lapse.
He first argues that all perceptions of the mind can be classed as impressions or ideas. He holds that a simple idea is always copied from an antecedent similar impression.
Almost immediately after saying this, he seems to produce an idea not derived from an impression, ‘one contradictory phenomenon’:
"Suppose… a person… perfectly well acquainted with colours of all kinds, excepting one particular shade of blue… which it has never been his fortune to meet with. Let all the different shades of that colour, except that single one, be plac’d before him, descending gradually from the deepest to the lightest… he will perceive a blank, where that shade is wanting, and will be sensible, that there is a greater distance in that place betwixt the contiguous colours, than in any other… I ask whether ’tis possible for him… to… raise up to himself the idea of that particular shade, tho’ it had never been convey’d to him by his senses?" (Treatise 18.104.22.168).
Yes he can, says Hume, and this ‘may serve as a proof’ that simple ideas are not always derived from an antecedent impression. However ‘this instance is so.. .singular, that ’tis scarce worth our observing, and does not merit that… we shou’d alter our general maxim’.
Over the years the exception has been variously explained away as,
* not really an exception: it depends on sensory experience of other shades of blue; or colours are complex not simple ideas; or the subject perceives it as conceptual content (he perceives ‘missing blue shadily’ as modern ‘adverbial’ accounts of perception have it), not as an image.
* Hume being paradoxical in hopes of better book sales (!)
* Hume being ironic. His declared method is observation/ experiment and the undoable thought experiment (how could I know whether I had or hadn’t previously seen that missing shade?), like the metaphysical speculation he decries, is not to be trusted.
But Hume is clear (and, it seems to me, serious): colours are simple ideas, the subject imagines (has an image of) the missing shade, it is an exception to his Principle; and he also appears to recognize that the instance generalizes to other colours and to other sensory modalities.
To me, the least implausible suggestion, is that Hume uses an exception to emphasize that his Principle is an empirical one. In his terminology, it is not a Relation of Ideas (an a priori truth) which would not necessarily tell us anything about the world, but rather a Matter of Fact (contingent truth) of which, therefore, the contrary is logically possible. He is speaking of a conceivable contradiction, not an actual one. But there again he could just have said this without any elaborate example.
So, Hume’s blue shade is a grey area, just right for scholarly dispute of the storm-in-a-teacup sort.