The doctrine of St. Thomas is that all knowledge is by ideas, in the sense of image, likeness, or similitude. In God the idea, image, likeness, or similitude, the _species_, is not distinguishable from the divine essence, for he is in his essence _similitudo omnium rerum_. Now, though we are created after the _idea exemplaris_, or model eternal in his essence, and therefore in our degree copy or imitate him, we have not in us the types or models of all things, are not in ourselves _similitudo omnium rerum_, and therefore are not intelligent in ourselves alone. The ideas by which things are intelligible and we intelligent must be distinct from us, and exist independent of us. As no creature any more than we has in itself the likeness of all things, or is in itself its own _idea exemplaris_, no creature can be in itself alone intelligible. Hence what the schoolmen call idea or intelligible species must be equally distinct from and independent of the object when the object is _aliquid creatum_, or creature. Hence, while both the created subject and the created object depend on the idea, the one to be intelligible, the other to be intelligent, the idea, intelligible species, the light--as we prefer to say--is independent of them both. The idea _in re_ is not something intermediary between subject and object, as is sometimes supposed, but the light that intervenes between them, as the necessary condition of knowledge in creatures. This seems to us to be the real doctrine of the scholastics, as represented by St. Thomas, and is, in our judgment, indisputable. {157} We call the idea, regarded as intervening in the fact of knowledge, the light, and thus avoid the question whether all knowledge is by similitude or not. It may be that the idea is light because it contains the image or likeness of the object, but that seems to us a question more curious than practically important. We cannot see that the explication of the mystery of knowing is carried any further by calling the idea image or similitude than by simply calling it the intelligible light. The Platonists and peripatetics seem to us to come no nearer the secret of knowledge by so calling it than do our philosophers to the secret of external vision, when they tell us that we do not see the visible object itself, but its image painted by the external light on the retina of the eye. How do I see the image or picture, and connect it with the external object? When I have called the object or the idea light, I seem to myself to have said all that can be said on the point, and to retain substantially the scholastic doctrine of ideas, or intelligible species, which asserts, I add, by the way, what is perhaps very true, but which after all brings us no nearer to the secret of knowledge, or the explanation of how in the last analysis we do or can know at all.