One of the main themes of the book is the beauty that mathematics possess, which Hardy compares to painting and poetry. For Hardy, the most beautiful mathematics was that which had no applications in the outside world, by which he meant pure mathematics, and, in particular, his own special field of number theory. He justifies the pursuit of pure mathematics with the argument that its very "uselessness" meant that it could not be misused to cause harm. On the other hand, Hardy denigrates applied mathematics, describing it as "ugly", "trivial" and "dull. "