According to Gilson, Abelard’s mistake was that he used logic to answer a philosophical question (which is the question of the Universals), and the answer that he got was a logical one and not a philosophical one. The consequence of this was that Abelard found that the question of the Universals is unanswerable and is like a pseudo-question. Here’s an excerpt from Gilson’s The Unity of Philosophical Experience (Chapter 1):
My point is that Abelard mistook logic for philosophy; but what about logic itself? Abailard was a logician trespassing on philosophical ground because, as they knew practically nothing else, the natural approach of twelfth-century men to philosophy was logic. Yet, before studying logic, they had always learned something else; namely grammar, with the unavoidable result that grammar was their normal approach to logic. The consequence of such a procedure was that Abelard was just as tempted to mistake grammar for logic as he was to mistake logic for philosophy. Now, what is the subject matter of grammar? It is language. Language itself is made up of words. It is the proper task of the grammarian to classify the various kinds of words of which our common speech is composed, to define their respective functions and to formulate the laws that determine their connections. As a distinct science—and it is for talking beings the most fundamental of all—grammar knows nothing but words. If you ask a grammarian a question, and if he answers it as a grammarian, your problem will inevitably be reduced by him to a mere question of words. Hence Abelard's famous sentence: "Now, however, that reasons have been given why things cannot be called universals, taken either singly or collectively, because they are not predicated of many, it remains to ascribe universality of this sort to words alone."Gilson is of the view that if Abelard had been “in a position to understand the import of that problem and to realize its specific nature, he would at last have discussed a philosophical problem in a philosophical way.” The ultimate result of Abailard’s mistake was skepticism.
In Chapter 2, “Theologism and Philosophy,” Gilson talks about the reason for which the philosophers of the Middle Ages became so enamored to the logical way. He points out that “when in the late eleventh and early twelfth centuries mediaeval men rediscovered logic, they became intoxicated with the wine of formal reasoning and the abstract beauty of its laws. Hence their natural tendency to deal in a purely logical way with all possible questions. They did this in philosophy and, as was to be expected, they did it also in theology.”