First, a significant mark that the human soul is rational is its difference from irrational creatures. This is why they are naturally called irrational, since the human race is rational.
Second, it is no small proof that the human being alone reasons about things outside himself, considers things that are not present, reflects further, and by judgment chooses the better among his thoughts. Irrational creatures see only what is present and rush only toward what is before their eyes, even if harm follows afterward. A human being, however, is not driven simply by what he sees; by reasoning he judges what he sees with his eyes. Often an impulse is checked by reasoning, and after reasoning he reflects again. Everyone who is a friend of truth can perceive that the human mind is distinct from the bodily senses.
Because it is distinct, the mind becomes judge of the senses. It distinguishes what they apprehend, calls it to mind, and shows them what is better. The eye’s function is only to see, the ears’ to hear, the mouth’s to taste, the nose’s to perceive smells, and the hands’ to touch. But determining what one ought to see and hear, what one ought to touch, taste, and smell, no longer belongs to the senses but to the soul and its mind. The hand can take hold of a sword, and the mouth can taste poison, but they do not know that these things are harmful unless the mind judges them.
To see this by an illustration, it is like a well-made lyre and a musician who holds it with skill. Each string of the lyre has its own sound—one low, another high, another in between, another still higher, and another different again. Yet without the skilled musician their harmony cannot be distinguished and their arrangement cannot be understood. Their harmony appears and their arrangement becomes right only when the one holding the lyre strikes the strings and touches each one in harmony. In the same way, when the senses are arranged in the body like a lyre and the skilled mind governs them, the soul then distinguishes and knows what it is doing and carrying out.
This alone is distinctive of human beings, and this is what is rational in the human soul. By using it, the soul differs from irrational creatures and shows that it is truly distinct from what is visible in the body. Often, while the body lies on the earth, a person imagines and contemplates things in the heavens. Often, while the body is still and at rest and asleep, the person moves inwardly and contemplates things outside himself, traveling to other lands, walking about, meeting acquaintances, and often through these experiences foreseeing and knowing beforehand what he himself will do by day. What else could this be but a rational soul, in which a person reasons about and understands things beyond himself?