Such are the words of the psychologist, DuBois-Reymond, in one of his well-known lectures. The agnostic attitude is the most seemly that it is possible to take. Nowadays, not only have all religious ideas been upset, but so too has everything which until now appeared most solid, most indivisible. Who has faith any longer in the atom? Who believes in the soul as a monad? Who believes in the objective validity of the senses?