(1) We must bear in mind that the attempt to make a critical examination into the foundations of our knowledge, and to determine its scope, is by no means a new thing.  Among the Greeks, Plato, Aristotle, the Stoics, the Epicureans, and the Skeptics, all attacked the problem.  It did not, of course, present itself to these men in the precise form in which it presented itself to Kant, but each and all were concerned to find an answer to the question:  Can we know anything with certainty; and, if so, what?  They may have failed to be thoroughly critical, but they certainly made the attempt.

I shall omit mention of the long series of others, who, since that time, have carried on the tradition, and shall speak only of Descartes and Locke, whom I have above brought forward as representatives of the two types of doctrine that Kant contrasts with his own.

To see how strenuously Descartes endeavored to subject his knowledge to a critical scrutiny and to avoid unjustifiable assumptions of any sort, one has only to read that charming little work of genius, the “Discourse on the Method of Rightly Conducting the Reason.”

In his youth Descartes was, as he informs us, an eager student; but, when he had finished the whole course of education usually prescribed, he found himself so full of doubts and errors that he did not feel that he had advanced in learning at all.  Yet he had been well tutored, and was considered as bright in mind as others.  He was led to judge his neighbor by himself, and to conclude that there existed no such certain science as he had been taught to suppose.

Having ripened with years and experience, Descartes set about the task of which I have spoken above, the task of sweeping away the whole body of his opinions and of attempting a general and systematic reconstruction.  So important a work should be, he thought, approached with circumspection; hence, he formulated certain Rules of Method.

“The first,” he writes, “was never to accept anything for true which I did not clearly know to be such; that is, carefully to avoid haste and prejudice, and to include nothing more in my judgments than what was presented to my mind so clearly and distinctly as to exclude all reason for doubt.”

Such was our philosopher’s design, and such the spirit in which he set about it.  We have seen the result above.  It is as if Descartes had decided that a certain room full of people did not appear to be free from suspicious characters, and had cleared out every one, afterwards posting himself at the door to readmit only those who proved themselves worthy.  When we examine those who succeeded in passing muster, we discover he has favored all his old friends.  He simply cannot doubt them; are they not vouched for by the “natural light”?  Nevertheless, we must not forget that Descartes sifted his congregation with much travail of spirit.  He did try to be critical.