In coming to know a mathematical truth visual experience can play a merely “enabling” role. For example, visual experience may have been a factor in a person’s getting certain concepts involved in a mathematical proposition, thus enabling her to understand the proposition, without giving her reason to believe it. Or the visual experience of reading an argument in a text book may enable one to find out just what the argument is, without helping her tell that the argument is sound. In earlier sections visual experience has been presented as having roles in proof and propositional discovery that are not merely enabling. On the face of it this raises a puzzle: mathematics, as opposed to its application to natural phenomena, has traditionally been thought to be an a priori science; but if visual experience plays a role in acquiring mathematical knowledge which is not merely enabling, the result would surely be a posteriori knowledge, not a priori knowledge. Setting aside knowledge acquired by testimony (reading or hearing that such-&amp;-such is the case), there remain plenty of cases where sensory experience seems to play an evidential role in coming to know some mathematical fact.