In one sense, of course, there’s less mystery in math than there is in any other human endeavor. In math we can really understand things, in a deeper way than we ever understand anything else. (When I was younger, I used to reassure myself during suspense movies by silently reciting the proof of some theorem: here, at least, was a certainty that the movie couldn’t touch.) So how is it that many people, including mathematicians, do feel there’s something “mysterious” about this least mysterious of subjects? What do they mean?