In his book, Spirit of Medieval Philosophy, twentieth century French Christian philosopher Étienne-Henry Gilson argues that in contradiction to the philosophic conventions of his time, the medieval period was not awash solely in religion, but also had its own philosophic tradition. According to Gilson's argument, particularly in the first and second chapters of his work, the Christian philosophy of the period was not, as has been argued, religious doctrine proffered as philosophy. Rather, it was a re-imagining of both Platonic and Aristotelian philosophy in light of the Christian worldview that resulted from what the medieval Christians believed was the divine revelation found in the Catholic Scriptures and magisterial Tradition.
Gilson first takes up the question of how any philosophy can be Christian. He argues that philosophy, as philosophy itself, must of necessity depend on a proper methodology for its use of reason in light of principles that are evident in themselves. According to Gilson, it follows that if a philosophy seems to be concordant with Christianity, particularly with the Catholic scriptural and magisterial tradition that was prevalent in the medieval period, then it must seem so not out of this Christian milieu, but rather because of its apparent truth independent of Christian influence. Gilson also raises the questions first posited by St. Augustine with respect to mingling philosophy, especially that originating from pre-Christian, pagan sources, with the doctrine and dogma of Christianity. With St. Augustine, the philosopher argues that any philosophy drawn from solely from pagan sources and superficially supplemented with Christian doctrine lacks an intrinsic Christian character. Its truth must arise from its philosophical content, from its proper use of reason and maxims of that are both logical and universally applicable. On the other hand, if the so-called Christian philosophy is considered to be veritable simply because it is Christian, then such philosophy would indeed cease being actual philosophy: it would, rather, be more accurately characterized as religion supported by arguments from reason.
Having taken pains in his first chapter to establish, with St. Augustine's help, what Christian philosophy is not, Gilson then takes up the task of describing what he believes it to be. While it is clear that Gilson's definition seeks to describe the realm of Christian philosophy during the medieval period, it can also be argued that his own context influences his analysis—he is providing a prescription for what he thinks Christian philosophy should be in his own day as well. One of the primary critiques against Christianity as a philosophy rather than as a religion is its fundamental provision of the means to an after-life, or in the terms of Christianity itself, salvation. According to Gilson, his contemporaries excluded, a priori, such considerations from their own philosophical theories. Because of this, Christianity, both in its medieval historical position and in Gilson's own time, has been excluded as a possible philosophy because of its metaphysical, salvific orientation. To answer this objection, Gilson raises the notion posited by St. Anselm, that philosophy should be fides quarens intellectum, faith seeking understanding. For both St. Anselm and Gilson, the “faith” component of the formulation is not some suspension of disbelief. But, rather, a willingness to engage with reason the arguments presented by Christianity. In this way, the fides that is presented is related to the way the word is used in the Anglo-American legal maxim, bona fides, good faith.
Using Anselm's philosophical thesis as a paradigm, Gilson sketches out a definition of Christian philosophy where “Christian” is a descriptive adjective rather than a categorical noun: in other words, he seeks to set out what Christian philosophies look like instead of attempting to describe a monolithic Christian Philosophy. Such a Christian philosophy, in the sense of what would make a philosophy truly Christian, Gilson argues would be one which keeps both the order of philosophy and the order of Christian religion distinct while at the same time utilized Christian revelation as an “indispensable auxiliary” to reason. Gilson's argument seems reasonable, particularly in light of the fact that he is positing a faith-and-reason approach to Christian philosophies to support his book's broader thesis that medieval European society did have philosophic endeavors, and not just religion dressed up by medieval theologians to appear to be philosophy.