He who is cruel to animals becomes hard also in his dealings with men. We can judge the heart of a man by his treatment of animals.

The sentiment fits Kant’s broader moral psychology: habits of feeling and action shape one’s character, and cruelty can erode the dispositions (sympathy, self-restraint) that support duties toward persons. Even though Kant is often read as denying animals direct moral standing (grounding duties primarily in rational agency), he does argue that cruelty to animals is morally corrosive because it damages the humanity in oneself and can spill over into how one treats other people. The quote, as commonly circulated, expresses that idea in a compact, character-judging aphorism.