The use of disagreement as a tool of reason appears to have been pervasive in Hellenistic culture (I leave the identification of its first discovery to historians.) It reached its highest refinement in Talmudic discussions of "disagreement for a higher purpose" ("machloket l'shem shamayim," lit. "disagreement in the name of the sky.") The "higher purpose" was to get closer to truth - in contrast to ordinary, polemical disagreement. The purpose of polemical disagreement is other-directed: to convert another to what one already believes to be true. The purpose of "high-minded" disagreement is inner-directed: what Ayn Rand would call "selfish," and her less frank contemporaries would describe as serving one's own "self-actualization" or "self-realization." The purpose of "higher" (that is, selfish) disagreement is to get one's own views into closer alignment with reality external to consciousness - that is, into closer agreement with what actually exists.
The reason for the selfish utility of intellectual disagreement is simple, and rather obvious. When two sane, intelligent, honest and knowledgeable persons disagree, it must be, that at least one of them knows something that the other does not know, or, that at least one of them has made an error of reasoning, or some combination of these. Because I know that I am neither omniscient nor infallible, and I because I want the content of my awareness to be aligned as closely as possible with what actually exists in the world, every disagreement with one of my intellectual peers presents me with what could be an opportunity to extend my knowledge, or to correct an error in my understanding of the world. Such a disagreement is a selfish opportunity that I, as a rational man, am not minded to waste.
It is unfortunate that we live in a Christian-Moslem-Altruist culture in which disagreement is viewed, mainly, as an other-directed opportunity to improve the other person by converting her to one's own, presumed-to-be-truer viewpoint. Intellectual disagreement is a useful tool of selfish reason, and deserves to be used for this, its eminently higher and more moral function.