Sir William Rothenstein was in Berlin doing a portrait of Einstein. The mathematician was always accompanied to the studio by a solemn, academic looking individual who sat in a corner throughout the sittings. Einstein, not wishing to waste any time, was putting forth certain tentative theories, to which the silent companion replied only by an occasional nod or shake of the head. When the work was concluded, Rothenstein, who was curious, asked Einstein who his companion was.
“That's my mathematician,” said Einstein, “who examines problems which I put before him and checks their validity. You see, I am not myself a good mathematician . . .”