Helen De Cruz in Bored Panda:
The thought experiment: A man has seen all colours, except one particular shade of blue. But he has seen other gradations of this colour, and if he were to arrange them in his mind, it would become clear that there’s a gap. Would he be able to fill in the color using his own imagination?
Significance: Hume came up with this thought experiment as a counterexample to his idea that we learn about the world through experience. If that’s the case, we should not be able to fill in the missing shade of blue but it seems we can. Curiously though, when I presented this drawing to friends, they thought the man’s sweater was the missing shade of blue, but it isn’t! So perhaps it is not so easy to fill in the gap after all.
Source: Hume, D.(1748). Philosophical essays concerning human understanding. London: A. Millar.