We get great pleasure from reading. The more advanced a man is, the greater delight he will find in reading. The ordinary man may think that subjects like philosophy or science are very difficult and that if philosophers and scientists read these subjects, it is not for pleasure. But this is not true. The mathematician finds the same pleasure in his mathematics as the school boy in an adventure story. For both, it is a play of the imagination, a mental recreation and exercise.

(Trích đề minh họa 2018)

What does the passage mainly discuss?