This makes mechanistic thinking different from just offering descriptions, and makes science different from engineering. Gears are all the same kind of part, but each gear has different properties: different radii, different numbers of teeth, and so on. He was appealing to hidden entities; so why wasn’t his alchemy a mature science? Darwinian entities and rules — entities with variation — is thus far too large a set to organize biology. “What Makes a Mature Science.” Asimov Press (2025).