3. Chap. II.--The mind is destitute of images presented by the imagination, so long as it has not been informed by the senses of external things.

In the second place, as to your opinion that it is possible for the mind to form to itself images of material things independently of the services of the bodily senses, this is refuted by the following argument:--If the mind is able, before it uses the body as its instrument in perceiving material objects, to form to itself the images of these; and if, as no sane man can doubt, the mind received more reliable and correct impressions before it was involved in the illusions which the senses produce, it follows that we must attribute greater value to the impressions of men asleep than of men awake, and of men insane than of those who are free from such mental disorder: for they are, in these states of mind, impressed by the same kind of images as impressed them before they were indebted for information to these most deceptive messengers, the senses; and thus, either the sun which they see must be more real than the sun which is seen by men in their sound judgment and in their waking hours, or that which is an illusion must be better than what is real. But if these conclusions, my dear Nebridius, are, as they obviously are, wholly absurd, it is demonstrated that the image of which you speak is nothing else than a blow inflicted by the senses, the function of which in connection with these images is not, as you write, the mere suggestion or admonition occasioning their formation by the mind within itself, but the actual bringing in to the mind, or, to speak more definitely, impressing upon it of the illusions to which through the senses we are subject. The difficulty which you feel as to the question how it comes to pass that we can conceive in thought, faces and forms which we have never seen, is one which proves the acuteness of your mind. I shall therefore do what may extend this letter beyond the usual length; not, however, beyond the length which you will approve, for I believe that the greater the fulness with which I write to you, the more welcome shall my letter be.