Is deduction a source of knowledge? Vailati on pragmatism and prediction

One the themes of classical Pragmatism, at least in Peirce's and Vailati's philosophy, is the cognitive role of deductive inference. Is deduction a source of knowledge? The answer seems to be in the positive when it is claimed that deductive inference draws the consequences of our hypotheses and brings them to the test of experience. I basically agree with this claim, but I also think that a careful analysis is needed on what we mean in this context by "source" and "knowledge" in particular. We must distinguish, as Vailati did, heuristic from epistemic value. Making deductive predictions does not amount to acquiring knowledge, but it certainly has heuristic value. So, deduction is not a source of knowledge if by "source" we mean a necessary or a sufficient condition of it, but it is a source of knowledge if we mean something like a heuristic device.