External objects present us only with appearances. Concerning them, therefore, we may be said to possess opinion rather than knowledge. The distinctions in the actual world of appearance are of import only to ordinary and practical men. Our question lies with the ideal reality that exists behind appearance. How does the mind perceive these ideas? Are they without us, and is the reason, like sensation, occupied with objects external to itself? What certainty could we then have, what assurance that our perception was infallible? The object perceived would be a something different from the mind perceiving it. We should have then an image instead of reality. It would be monstrous to believe for a moment that the mind was unable to perceive ideal truth exactly as it is, and that we had not certainty and real knowledge concerning the world of intelligence. It follows, therefore, that this region of truth is not to be investigated as a thing external to us, and so only imperfectly known. It is _within_ us. Here the objects we contemplate and that which contemplates are identical,—both are thought. The subject cannot surely _know_ an object different from itself. The world of ideas lies within our intelligence. Truth, therefore, is not the agreement of our apprehension of an external object with the object itself. It is the agreement of the mind with itself. Consciousness, therefore, is the sole basis of certainty. The mind is its own witness. Reason sees in itself that which is above itself as its source; and again, that which is below itself as still itself once more. Knowledge has three degrees—Opinion, Science, Illumination. The means or instrument of the first is sense; of the second, dialectic; of the third, intuition. To the last I subordinate reason. It is absolute knowledge founded on the identity of the mind knowing with the object known.[18]