Scepticism

2. Responses to scepticism

This argument presents problems for fallibilism, as I have characterized it, since the argument at no point presupposes the entailment principle. The sceptical argument we are now considering merely exploits the fallibilist position that permits the existence of alternatives to known propositions.

Fallibilist responses come in two forms, each of which corresponds to the denial of one of the two premises of the sceptical argument. One response denies the closure principle. For example, Dretske (1970) has argued that the fact that we do not know the falsity of sceptical alternatives shows that the closure principle is false, since we do know the truth of many empirical propositions that (we know) entail the falsity of sceptical alternatives. According to this view, certain alternatives are not relevant to whether one knows a proposition: one does not have to know such an alternative to q is false in order to know q. So, for example, one can know that one sees a zebra without knowing that the alternative – that one sees a cleverly disguised mule – is false, because that alternative is not relevant. This version of fallibilism is sometimes called the ‘relevant alternatives’ view.

The other fallibilist response to the sceptical argument agrees with the sceptic that the closure principal is true. But, against the sceptic, these fallibilists deny the claim that we fail to know the falsity of sceptical alternatives. One version of this fallibilist response uses the closure principle along with the claim that we do have knowledge, to reject the claim that we do not know that sceptical alternatives are false. They argue from the premise that we know some ordinary proposition q and the premise that if we know q then we know any proposition that we know is entailed by q (the closure principle), to the conclusion that we know that we are not seeing a cleverly disguised mule. We can call this view ‘modus ponens fallibilism’.