Horn, wood, stone, sowing, and harvest

Meaning

The Way of Truth compares using arbitrarily chosen substances to sowing horn, wood, or stone and expecting a harvest of grain. The force is not that all transformation is impossible, but that the expected result must have a fitting natural ground: the speaker locates the art in the essence of the natures, not in any imagined starting substance. The dialogue's alchemist independently warns sophists not to reap where they have sown nothing. Both agricultural expressions criticize an unjustified expectation of results. Neither gives a rule that every mineral reproduces like a seed-bearing plant, a proposition the dialogue expressly disputes elsewhere.