Hume considers an objection to his thesis based on a possibility of generating an idea of an unexperienced shade of color by interpolation. [text]
There are two very different colors (e.g. a particular shade of red (call it red-208) and a particular shade of green (call it green-436)) such that there is a virtual continuum of shades between them; that is, there is a shade very close to the red and one very close to that and one very close to that . . . and one very close to that which is very close to the particular shade of green. [text] [critique]
A person (call him Blake) who has been seeing colors for many years, but has never seen a certain particular shade of blue (call it blue-227) if presented with all other shades of blue arranged by shade will perceive that there is a shade missing at that point (between blue-226 and blue-228). [text] [critique]
Blake (as described in premise 3) will be able to generate an idea of blue-227 by imagination even though he never had an impression of blue-227. [text] [critique]
Ideas of colors and of each particular shade of a color are all simple ideas. [footnote] [critique]
Therefore: [flow] [Critique]
The thesis is false.
1. I ignore the claim about sounds because the remainder of the argument appeals to the claim about colors.
2. This premise is not explicitly stated by Hume, but is necessary for the argument and seems to be presumed since Hume states as the conclusion of this argument "that simple ideas are not always in every instance, derived from the correspondent impressions . . ." [text]