"What Is Man?" is a short story by American writer Mark Twain, published in 1906. It is a dialogue between a Young Man and an Old Man regarding the nature of man. The title refers to Psalm 8:4, which begins "what is man, that you are mindful of him...".
It involves ideas of determinism and free will, as well as of psychological egoism. The Old Man asserts that the human being is merely a machine, and nothing more, driven by the singular purpose to satisfy his own desires and achieve peace of mind. The Young Man objects and asks him to go into particulars and furnish his reasons for his position.