Abstract

Both these ideas are intuitively plausible: rationality has an external aim, such as forming a true belief or good decision; and the rationality of a belief or decision is determined purely by facts about the thinker’s internal mental states. Unlike earlier conceptions, the conception of rationality presented here explains why these ideas are both true. Rational beliefs and decisions, it is argued, are those that are formed through the thinker’s following ‘rules of rationality’. Some rules count as rules of rationality because it is rational to believe---through following other rules---that those rules are reliable. But there must also be certain basic rules, which are a priori, or ‘built into’ our basic cognitive capacities. That these rules are a priori is a purely internal matter; and in following these rules the thinker has done all that could reasonably be expected to achieve the external aim of forming a true belief or good decision.