“One attempt to avoid the problem of induction involves weakening the demand that scientific knowledge be proven true, and resting content with the claim that scientific claims can be shown to be probably true in the light of the evidence. So the vast number of observations that can be invoked to support the claim that materials denser than air fall diWInwards on earth, although it does not permit us to prove the truth of the claim, does warrant the assertion that the claim is probably true.”