If an ear mistakes a sound or the eye a colour, the aurist, or the oculist, at once infers that the organ is unhealthy; as sound is the "proper object" of hearing, and colour of sight. But a mistake about the direction of a sound, or the distance of a hill, shows, not an unhealthy, but an untrained ear or eye; as direction and distance are "accidental objects" of hearing and sight. In like manner the understanding in health, or the normal understanding, never errs when it says, 'Here's something': this is the cognition of "abstract being," the "proper object " of the understanding.

Of God and His Creatures: 1.61