In the prolegomena (as it were) to the Summa, Aquinas distinguishes between the kinds of authority proper to (1) divine revelation as present in Scripture, (2) reason, and (3) tradition (that is, the kinds of authority proper to (2) the philosophers and (3) the doctores ecclesiae):

But sacred doctrine makes use even of human reason, not, indeed, to prove faith (for thereby the merit of faith would come to an end), but to make clear other things that are put forward in this doctrine. Since therefore grace does not destroy nature but perfects it, natural reason should minister to faith as the natural bent of the will ministers to charity. Hence the Apostle says: “Bringing into captivity every understanding unto the obedience of Christ” (2 Corinthians 10:5). Hence sacred doctrine makes use also of the authority of philosophers in those questions in which they were able to know the truth by natural reason, as Paul quotes a saying of Aratus: “As some also of your own poets said: For we are also His offspring” (Acts 17:28). Nevertheless, sacred doctrine makes use of these authorities as extrinsic and probable arguments; but properly uses the authority of the canonical Scriptures as an incontrovertible proof, and the authority of the doctors of the Church as one that may properly be used, yet merely as probable. For our faith rests upon the revelation made to the apostles and prophets who wrote the canonical books, and not on the revelations (if any such there are) made to other doctors. Hence Augustine says (Epis. ad Hieron. xix, 1): “Only those books of Scripture which are called canonical have I learned to hold in such honor as to believe their authors have not erred in any way in writing them. But other authors I so read as not to deem everything in their works to be true, merely on account of their having so thought and written, whatever may have been their holiness and learning.” (ST I, Q. 1, Art. 8, ad. 2)

Here we see a principled difference between the canonical Scriptures and all other authorities: only the “authority of the canonical Scriptures” can be used as “incontrovertible proof.” One may of course use the writings of the Fathers or other theologians, “yet merely as probable”: there is no guarantee that they are always right.

This distinction is of absolutely fundamental importance, for it means that there is, for Thomas, a principled difference between the canon and its interpretation, howsoever lofty the interpreter. In the passage above there is no blurring of the line between the Word of God and the word of man, such that we can only know what the former says from its reception in the latter. The difference between certain and probable authority points, then, to an essential difference between Scripture itself and the activity of reading, interpreting, or expounding Scripture.

The authority of Scripture as incontrovertible proof over against the probable authority of ecclesiastical doctors therefore necessitates, in the nature of the case, a critical relation to tradition, the Fathers, and so on: since their authority is only “probable,” there is a principled space–again, in the nature of the case–for judgment of and dissent from it.

Augustine serves as one of those “probable” authorities Thomas can use to make his point. In his bid for responsible reading (and the implicit self-effacing stance toward his own writings–and those, to be sure, of Jerome!), Augustine makes exactly the distinction Thomas does. Notice the import of the adverb and the result clause (“other authors I so read as not to deem everything in their works to be true”): non-canonical authors are to be read in such a way that one does not assume at the outset that everything in them is true; they are not infallible. We should mark well what Augustine’s proposal means, for it requires both the possibility and the necessity of critical reading–not of captiousness and carping, but nevertheless of personal responsibility in the appropriation of the word of man as an elucidation of the Word of God.

The proposal further requires–yet a third time, in the nature of the case– a standard by which to judge all other theologians and authorities. What is it? What does our faith rest upon? “Our faith,” Thomas says, “rests upon the revelation made to the apostles and prophets who wrote the canonical books.” It rests, in other words, on the fundamentum of divine revelation as given final form in Scripture, theology’s principium cognoscendi externum.