"Kant has said that there are some questions which should never be asked, and there is apostolic authority for the injunction "to avoid foolish questions." Only the fool has said in his heart that he is alone in the universe; but since philosophy has seriously raised the question of the existence of my neighbor and of the way in which I may come to know him, it may be not without interest to notice (i) how the problem has emerged, (ii) the importance of the problem for modern philosophy, and (iii) some leading solutions that have been offered."