Letter 121 — On Instinct in Animals (§11)

“Your objection would be true if I spoke of living creatures as understanding “a definition of constitution,” and not “their actual constitution.” Nature is easier to understand than to explain; hence, the child of whom we were speaking does not understand what “constitution” is, but understands its own constitution. He does not know what “a living creature” is, but he feels that he is an animal.”