He took a psychological fact and applied it to the functioning of the real world, with the result that he posited an absurd picture of the world and one that, for obvious reasons, led to skepticism. Skepticism, Gilson repeatedly illustrates, is the ultimate result of all the isms. Montaigne s skepticism, in turn, spurred Descartes to find a different source of truth, which led to his mathematicism, spiritualism, and idealism, which eventually broke down and led to the skepticism of Hume. Both of these isms, in turn, contributed to the widespread skepticism that is our lot today. This is a consistent trait of some of his most well known books, like Reason and Revelation in the Middle Ages, God and Philosophy , and The Spirit of Mediaeval Philosophy .
Unity bears the same trait.
In one sentence, one of the biggest issues in philosophical history was illuminated immediately. This isn t to say that the book can be read lightly. Far from it. It must be read cautiously and purposefully. The prose is good and clear and therefore understandable, but the topics are nonetheless complex.