Consider, first, the general trend of medieval philosophy. Here, by and large, was a philosophy that has since come to be known as "realist." The mind, it was admitted without questioning, was ruled by a world of objects that exist apart from it and govern its content - "measured the intellect," in Scholastic idiom. Being, or reality, came first; thought, corresponding to reality, came second. Interpreted, this means that what the mind knows directly is reality; then, upon reflection, its thought of reality. Transposed to sense it means that when I look at a thing, what I see is the thing and not my sight of the thing. This is the attitude of common sense, and both the ancients and medievals as a whole, accepted it on that basis; it was, I mean, the most natural and obvious attitude to take - whether this makes them naive and uncritical we need not inquire. They did not, however, pretend that the question of the validity of knowledge was thereby settled. The truth of the matter is that the so-called objectivity (realism) of knowledge did not figure prominently in their speculations, since no acknowledged philosopher seriously doubted the common-sense view. And thus affairs stood until the dawn of modern philosophy.
Prophetic of the new order was Descartes. With him, as with his posterity, the first immediate object of certain, that is, indubitable knowledge is no longer external reality. But if not reality, what then? Thought, or the activity of thought, is adjudged the more immediate experience, hence the first direct object of knowledge. The effect of this inversion was to leave the mind cut off from reality. Descartes, it is true, felt that you had only to invoke his basic intuition "I think, therefore I am," and you found yourself returned to reality. His successors, however, would not be assured; to them, a return to reality from knowledge that was not in the first instance moored to reality seemed highly problematical, and was indeed impossible, the idealist postulate having seen to that. For, as this postulate would have it, what you know directly is what goes on in your mind - external reality, if known at all, is known as an inference, never as something actually experienced. Thought, accordingly, lies immured, its contact with outside reality irretrievably lost. On this common theme of primacy of thought over external reality, subjectivists and idealists of every description fathered forth tractates and treatises of endless variety. On one thing, though, all agreed: there could be no true philosophy that was not based on the idealist postulate.26
Obviously, the supporters of traditional philosophy could not remain indifferent to this development. Threatened, in effect, was the whole structure of their philosophy; and thrust upon them was the question: should they persist in regarding extramental reality as the unexceptionable starting point of philosophy, or should they yield to the moderns, adopting the reflective study of knowledge as their springboard, after which they would again take their stand with the metaphysics of realism?
Offhand, there appears no reason why a follower of St. Thomas should not undertake a critique of knowledge, a systematically conducted inquiry into its nature and validity, especially its validity. Not a few eminent Thomists have done just that. But all attempts are well advised of certain restrictions. The critique should not be considered the indispensable preliminary to metaphysics, nor be allowed to supersede metaphysics as the superior wisdom; but above all it must from the outset avoid the idealist principle of absolute interiority, whereby thought is arbitrarily so impounded in the mind as to render direct access to reality impossible. On these conditions, an epistemology (a critique of knowledge) according to the mind of St. Thomas is quite possible and perhaps feasible.
Yet, when all is said, the true embodiment of wisdom still lies in a metaphysics that is at once realist and critical (self-critical, that is). St. Thomas states flatly that there is but one supreme science, and as supreme it does not hand over the task of justifying or defending its principles to another; it does it itself. "Metaphysics [sc. itself]," to quote his own words, "carries the argument to those who deny its principles." 27 This science must be grounded in realism, which is to say in being, assuming, as we do, that being is the first thing known by the intellect and constitutes its proper object. Although it takes its stand on realism, this science will also be, in fact cannot but be critical (or criterioligical); for there are difficulties encountered as to the validity of knowledge, very substantial difficulties, which must be met by the science itself, there being none higher to which they can be referred for solution. Granted that the difficulties in question may be organized into a separate study, outside the context of metaphysics proper and under the heading, as is sometimes done, of epistemology or criteriology; nevertheless, there seems more to be gained, and more logic as well, in conducting the critique of knowledge within the compass of metaphysics proper. If nothing more, such a procedure does better to safeguard the unity of metaphysics; and better kept too will be the autonomy, the complete self-jurisdiction that belongs to it as first philosophical wisdom. In this, moreover, we have Aristotle's precedent, who incorporates a considerable criteriological section into his metaphysics, in the course of which he defends the first principles of thought against the subjectivists of his time.28 We, for our part, shall follow Aristotle's example, so that after the next chapter, which is the metaphysical study of being, will follow a criteriological chapter, the critical examination of our knowledge of being.