Full Idea
Vice and virtue may be compared to sounds, colours, heat and cold, which, according to modern philosophy, are not qualities in objects but perceptions in the mind.
Gist of Idea
Virtues and vices are like secondary qualities in perception, found in observers, not objects
Source
David Hume (Letters [1739], to Hutcheson 1740)
Book Reference
'British Moralists 1650-1800 Vol. 2', ed/tr. Raphael,D.D. [Hackett 1991], p.110
A Reaction
Very revealing about the origin of the is/ought idea, but this is an assertion rather than an argument. Most Greeks treat value as a primary quality of things (e.g. life, harmony, beauty, health).