In 1900 the German mathematician David Hilbert delivered a lecture in Paris in which he laid out the problems he thought mathematics might deal with in the coming century. Before getting to them he said what he thought were the general criteria which mark a good one. He demanded first clarity and ease of comprehension, but then went on to say that such a problem‚ should be difficult in order to entice us, yet not completely inaccessible, lest it mock at our efforts.