He shows how men assist each other to know the truth; for one man assists another to consider the truth in two ways--directly and indirectly.
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.
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.
St. Thomas Aquinas, Commentary on Aristotle's Metaphysics, John P. Rowan, tr. Dumb Ox (Notre Dame: 1961) p. 110 (Book 2, Lesson 1; Section 287).