There are many examples of diagrams in which one seems to perceive necessity – one sees not only that something is so, but that it must be so. That conflicts with some well-known philosophical theses, inherited from Hume, according to which there cannot be any “necessary connections between distinct existences” to be perceived; and even if there were, perception would not be capable of gaining access to them. We defend the perception of necessity, and explain why Hume fails to show its impossibility.