The Roman mythological god Janus was said to bring all things into the world, from the seasons to the ways of civilization. He was symbolized as a two-faced god, one youthful, the other elderly. The authors find here an allegory to the apprentice-expert continuum in knowledge acquisition, so they've named a principle of human-centered computing after Janus. This principle deals with the distinction between performance and training, and its implications for intelligent technologies.