[Tesla’s] mental constructs were built with meticulous care as concerned size, strength, design and material; and they were tested mentally, he maintained, by having them run for weeks—after which time he would examine them thoroughly for signs of wear. Here was a most unusual mind being utilized in a most unusual way. If he at any time built a “mental machine,” his memory ever afterward retained all of the details, even to the finest dimensions.
James O’Neill, Prodigal Genius: The Life of Nikola Tesla, New York, 1944, pp. 51-52