Intelligent Design and Probability Reasoning

نویسنده

  • Elliott Sober
چکیده

This paper defends two theses about probabilistic reasoning. First, although modus ponens has a probabilistic analog, modus tollens does not – the fact that a hypothesis says that an observation is very improbable does not entail that the hypothesis is improbable. Second, the evidence relation is essentially comparative; with respect to hypotheses that confer probabilities on observation statements but do not entail them, an observation O may favor one hypothesis H1 over another hypothesis H2 , but O cannot be said to confirm or disconfirm H1 without such relativization. These points have serious consequences for the Intelligent Design movement. Even if evolutionary theory entailed that various complex adaptations are very improbable, that would neither disconfirm the theory nor support the hypothesis of intelligent design. For either of these conclusions to follow, an additional question must be answered: With respect to the adaptive features that evolutionary theory allegedly says are very improbable, what is their probability of arising if they were produced by intelligent design? This crucial question has not been addressed by the ID movement. Philosophers schooled in the rules of deductive logic often feel that they can find their way when reasoning about probabilities by using the idea that probability arguments are approximations of deductively valid arguments. In a deductively valid argument, the premisses necessitate the conclusion; in a strong probability argument, the premisses confer a high probability on the conclusion. As a probability argument is strengthened, the probability of the conclusion, conditional on the premisses, increases; in the limit, the premisses confer a probability of unity on the conclusion. Deductive validity thus seems to be the limit case of strong probability arguments. There is nothing wrong with this idea, though it does require refinement. However, there is a distinct though closely related thought that can lead one very much astray. This is the idea that for each deductively valid form of argument, there exists a strong probabilistic argument that has roughly the same form. Granted, this principle is vague as stated, but nonetheless I think it plays a heuristic role for many philosophers (and nonphilosophers also). I want to explain why there are fundamental reasons why this heuristic is not to be trusted. I’ll begin with an example in which the principle does no harm. Modus ponens has the following logical form:

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تاریخ انتشار 2002