His own unseeable thoughts, by which he argues against the existence of the invisible. Nobody has ever seen steam.
Electricity is invisible.
The science of physics is occultism. Experts in the uses of steam and electricity are sorcerers. Mostly we do not think of their practices as witchcraft, but we have an opinion upon what would have been thought of them, in earlier stages of the Dark Age we're living in. A neighbor had pigeons, and the pigeons loafed on my window sill. They were tempted to come in, but for weeks, stretched necks, fearing to enter.