让逻辑得其所：理性信仰的逻辑局限.doc

Putting Logic in its Place Formal Constraints on Rational Belief Christensen, David, Department of Philosophy, University of Vermont Abstract: Are rational beliefs constrained by formal logic? This book argues that if beliefs are seen in a binary way (either one believes a proposition or one doesnt), then the standard constraints of ’deductive cogency’ (i.e. deductive consistency and deductive closure) are incompatible with epistemic rationality. If, however, beliefs are seen as graded, or coming in degrees, a probabilistic constraint (based on standard logic) is imposed by ideal rationality. This constraint, probabilistic coherence, explains both the appeal of the standard deductive constraints and the power of deductive arguments. Moreover, it can be defended without taking degrees of belief (as many decision-theoretic philosophers have) to be somehow defined or constituted by preferences. Although probabilistic coherence is humanly unattainable, this does not undermine its normative status as a constraint in a suitably idealized understanding of epistemic rationality. Contents 1 LOGIC AND RATIONAL BELIEF 1 2 TWO MODELS OF BELIEF 12 3 DEDUCTIVE CONSTRAINTS: PROBLEM CASES, POSSIBLE SOLUTIONS 33 4 ARGUMENTS FOR DEDUCTIVE COGENCY 69 5 Logic, Graded Belief, and Preferences 106 6 Logic and Idealization 143 PREFACE When people talk informally about belief, “rational” and “logical” are often used almost synonymously. And even those who think carefully and precisely about rational belief often take logic to play an important role in determining which beliefs are rational. Explaining the importance of logic to students, philosophers often say things like, “Rational beliefs must be logically consistent with one another,” or “If you believe the premises of a valid argument, then, if you are rational, you must believe the conclusion.” This book aims to show that logic does indeed play an important role in characterizing ideally rational belief, but that its role is quite dif