The mathematician requires tact and good taste at every step of his work, and he has to learn to trust to his own instinct to distinguish between what is really worthy of his efforts and what is not; he must take care not to be the slave of his symbols, but always to have before his mind the realities which they merely serve to express.

Report of the Sixtieth Meeting of the British Association for the Advancement of Science, Presidential address (p. 725), John Murray. 1891