Full Idea
We may say "From P and If-P-then-Q, infer Q" (modus ponens), but there is no rule of acceptance to say that we should accept Q. Maybe we should stop believing P or If-P-then-Q rather than believe Q.
Gist of Idea
You don't have to accept the conclusion of a valid argument
Source
Gilbert Harman (Thought [1973], 10.1)
Book Reference
Harman,Gilbert: 'Thought' [Princeton 1977], p.157