He worked, re-worked, fine-tuned, and perfected inventions in his mind, thereby solving the engineering problems well before he ever turned these mental constructs into reality. He believed it was a total waste of time, money and energy to carry out and test a crude idea without first an intense visualization stage.
Today we use computer simulations for that. He was using his mind, as a computer, that did the simulated trial runs, to avoid the excess cost of experimental runs.