Full Idea
In many writers I find that instead of the usual propositions 'is' and 'is not', I then find no proposition that is not connected with an 'ought' or an 'ought not'. It is necessary that a reason be given for how one can be a deduction from the other.
Gist of Idea
You can't move from 'is' to 'ought' without giving some explanation or reason for the deduction
Source
David Hume (Treatise of Human Nature [1739], III.1.1)
Book Reference
Hume,David: 'A Treatise of Human Nature', ed/tr. Mossner,Ernest C. [Penguin 1969], p.521
A Reaction
A huge claim, the basis of the value-free modern scientific world view. Possible escapes are Greek virtue theory, or Kantian principles, or some sort of a priori values (as in Charles Taylor).