Full Idea
Conservatism is important; you should continue to believe as you do in the absence of any special reason to doubt your view, and in reasoning you should try to minimize change in your initial opinions in attaining other goals of reasoning.
Gist of Idea
Reason conservatively: stick to your beliefs, and prefer reasoning that preserves most of them
Source
Gilbert Harman ((Nonsolipsistic) Conceptual Role Semantics [1987], 12.2.6)
Book Reference
Harman,Gilbert: 'Reasoning Meaning and Mind' [OUP 1999], p.217
A Reaction
One of those principles like Ockham's Razor, which feels right but hard to justify. It seems the wrong principle for someone who can reason well, but has been brainwashed into a large collection of daft beliefs. Japanese soldiers still fighting WWII.