"One is assisted directly by those who have discovered the truth; because, as has been pointed out, when each of our predecessors has discovered something about the truth, which is gathered together into one whole, he also introduces his followers to a more extensive knowledge of truth.
"One is assisted indirectly insofar as those who have preceded us and who were wrong about the truth have bequeathed to their successors the occasion for exercising their mental powers, so that by diligent discussion the truth might be seen more clearly.
"288. Now it is only fitting that we should be grateful to those who have helped us attain so great a good as knowledge of the truth. Therefore he says that 'It is only right that we should be grateful,' not merely to those whom we think have found the truth and with whose views we agree by following them, but also to those who, in the search for truth, have made only superficial statements, even though we do not follow their views; for these men too have given us something because they have shown us instances of actual attempts to discover the truth. By way of an example he mentions the founders of music; for if there 'had been no Timotheus,' who discovered a great part of the art of music, we would not have many of the facts that we know about melodies. But if Timotheus had not been preceded by a wise man named 'Phrynis,' he would not have been as well off in the subject of music. The same thing must be said of those philosophers who made statements of universal scope about the truth of things; for we accept from certain of our predecessors whatever views about the truth of things we think are true and disregard the rest. Again, those from whom we accept certain views had predecessors from whom they in turn accepted certain views and who were the source of their information."
St. Thomas Aquinas, Commentary on the Metaphysics of Aristotle II.1.Comm nos. 15-16 =287-288 cumulative, trans. John P. Rowan (St. Thomas Aquinas, Commentary on the Metaphysics of Aristotle, trans. John P. Rowan, Library of living Catholic thought (Chicago: Henry Regnery Company, 1961), vol. 1, p. 119, as reproduced at http://dhspriory.org/thomas/Metaphysics2.htm).
The Latin (taken from the Marietti edition of 1950) is here: http://www.corpusthomisticum.org/cmp02.html#81853.
I was put on to this by Jean-Pierre Torrell, "Saint Thomas et l'histoire: état de la question et pistes de recherches," Nouvelles recherches thomasiennes, Bibliotheque thomiste 61, ed. L.-J. Bataillon, O.P, and A. Oliva, O.P. (Paris: Librairie philosophique J. Vrin, 2008), 151 (131-175) =Revue thomiste 105 (2005): 355-409.
Torrell goes on to cite also Commentary on the Metaphysics XII.9.Comm no. 14 =2566 cumulative:
"And since in choosing or rejecting opinions of this kind a person should not be influenced either by a liking or dislike for the one introducing the opinion, but rather by the certainty of truth, he therefore says that we must respect both parties, namely, those whose opinion we follow, and those whose opinion we reject. For both have diligently sought the truth and have aided us in this matter. Yet we must 'be persuaded by the more certain,' i.e., we must follow the opinion of those who have attained the truth with greater certitude"