Letter 58 — On Being (§16)

“But I now return to the subject which I promised to discuss for you, namely, how it is that Plato divides all existing things in six different ways. The first class of “that which exists” cannot be grasped by the sight or by the touch, or by any of the senses; but it can be grasped by the thought. Any generic conception, such as the generic idea “man,” does not come within the range of the eyes; but “man” in particular does; as, for example, Cicero, Cato. The term “animal” is not seen; it is grasped by thought alone. A particular animal, however, is seen, for example, a horse, a dog.”