Logic

Why is question-begging considered a fallacy when it embodies a deductively valid form of reasoning?

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Perhaps this just reflects that the notion of fallacy (in the broad sense) is used in a fairly catch-all way. Let's say (as a first shot) that a fallacy is someflaw in the structure of an argument which prevents its givingrationally persuasive support for its conclusion. And let's distinguish that from the narrower idea of a deductive fallacy, a feature of a supposedly deductively valid inference which prevents its being so.

Deductive fallacies are fallacies in the broad sense. But many fallacies in the broad sense are of course not deductive fallacies -- for example, fallacies in various kinds of inductive reasoning. The case of a "question-begging" argument like "P, therefore P", is another sort of case where the argument doesn't involve a deductive fallacy, but the structure prevents the argument giving any rationally persuasive support for its conclusion: so it is deemed to be a fallacy in the broad sense.

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