Full Idea
Suppose a person to be perfectly acquainted with all colours, except one particular shade of blue. It must be possible for him to raise up from his own imagination the idea of that particular shade, though never conveyed to him by the senses.
Gist of Idea
If a person had a gap in their experience of blue shades, they could imaginatively fill it in
Source
David Hume (Enquiry Conc Human Understanding [1748], II.16)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.21
A Reaction
[compressed] He dismisses this as 'so singular it is scarcely worth observing', but it is crucial. It isn't 'singular'. We do it all the time, by extrapolating from experiences and interpolating between them. Thus we extend knowledge beyond experience.
Related Ideas
Idea 11951 Hume allows interpolation, even though it and extrapolation are not actually valid [Molnar]
Idea 24034 If someone had only seen the basic colours, they could deduce the others from resemblance [Descartes]