Full Idea
One might have the idea that the unthinkable was itself a moral category. ...Regarding certain things even as alternatives is itself something to be regarded as dishonourable or morally absurd.
Gist of Idea
Maybe the unthinkable is a moral category, and considering some options is dishonourable or absurd
Source
Bernard Williams (A Critique of Utilitarianism [1973], 2)
Book Reference
Smart,J./Williams,B.: 'Utilitarianism For and Against' [CUP 1978], p.92
A Reaction
He's very tentative about this, but I think it is a powerful moral idea. See Kekes. He is particularly aiming at utilitarians, who happily assess vile possibilities.
Related Ideas
Idea 519 One must avoid even speaking of evil deeds [Democritus (attr)]
Idea 20151 Our attitudes include what possibilities we value, and also what is allowable, and unthinkable [Kekes]