Kant's formula of universal law provides two tests — contradiction in conception and contradiction in will — for evaluating the rationality of maxims.

The third argument claims that a passion is an “original existence”, not an idea, or a mental copy of another object. Contradiction to truth and reason “consists in the disagreement of ideas, consider’d as copies, with those objects, which they represent” (T 2.3.3.5). Therefore, a passion cannot be contrary to truth and reason. Passions cannot, strictly speaking, be evaluated as reasonable or unreasonable, despite our practice of calling passions unreasonable or irrational when they depend in so