MATHEMATICS is a curious subject. Though often classed as one, it is not really a science. That scientists use it to describe their interpretation of reality is not quite the same thing. Nor, though, is it an art—not, at any rate, in the modern meaning of that word. The aesthetics of the subject, which any mathematician will tell you are the driving force behind his passion, are not obvious to the senses in the way that those of a painting, a symphony or a play are. Yet Benoît Mandelbrot’s celebrity beyond the academy is largely due to art in its modern, sensuous, sense. For the “set” to which he gave his name, when computed, drawn on a complex plane and suitably tinted, appealed greatly to the senses—as a million posters, greetings cards and T-shirts, bought by people who had not the faintest idea what it was, attest.