國立臺灣大學哲學論評

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Validity is an orthodox way to distinguish between good and bad inferences, which says that when inferences are valid, we can derive true conclusions from true premises. But when inferences involve conditionals, validity does not seem to be a good criterion. Unsatisfied with the traditional view of validity, Adams(1965) and Stalnaker(1975) propose their modified criteria, which are called “reasonable inference” in this paper, to evaluate inferences involving conditionals. In this paper, Adams’ criterion is called “justification of assertibility”, which can derive a highly assertible conclusion from highly assertible premises. Stalnaker’s criterion is called “justification of strictness” in this paper, which can derive a strictly assertible conclusion from strictly assertible premises.

This paper examines these two criteria and their consequences. First, by reexamining McGee’s(1985) counterexample to modus ponens, I argue that McGee’s example is an assertibility-justified but not a strictness-justified counterexample to modus ponens. Second, I explain why these two criteria have such a substantial divergence when inferences have conditional conclusions. Third, if my diagnosis is correct and complicated conditionals are allowed in inferences, then the justification of assertibility is too rigid to be satisfied. I conclude that the justification of strictness is a better criterion than the justification of assertibility.