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.
How we do or can know seems to us an inexplicable mystery, as is our existence itself. That we do know is certain. Every man knows, and in knowing knows that be knows; but how he knows no man knows. To deny is as much an act of reason as is to affirm, and no one can deny without knowing that he denies. Men may doubt many things, but universal doubt is a simple impossibility, for whoever doubts knows that be doubts, and never doubts that he doubts or that doubt is doubting. In all things and in all science we arrive at last, if we think long and deep enough, at a mystery which it is in no human power to deny or to explain, and which is explicable only in God by his divine science. Hence it is that philosophy never fully suffices for itself, and always needs to be supplemented by revelation, as nature to attain its end must not only be redeemed from the fall, but supplemented by grace. Man never suffices for himself, since his very being is not in himself; and how, then, shall philosophy, which is his creation, suffice for itself? Let philosophy go as far as it can, but let the philosopher never for a moment imagine that human reason will ever be able to explain itself. The secret as of all things is in God and with him. Would man be God, the creature the Creator?