Our vocabulary has at its disposal two words : « faith » and « reason. » And since we are quick to move from the existence of words to the existence of things, common theory tells us first that we are endowed with reason, and secondly, that faith can be awakened in us, and that these are two modes of distinct but complementary knowledge. Let us explain. Common theory (which, like all common theories, is a recent theory) first depends upon an affirmation as old as philosophy, which is a philosophical affirmation : man defines himself specifically by the logos, which is called ratio in Latin ; whence « reason » and « rationality. » In its Greek origins, and once it became Roman, rationality is unlimited. As a result, there is one question that is never raised : that of an act of knowledge in which man exceeds his definition of « rational animal. » The question did arise in the Christian world, however, when the knowledge of God and divine things came to be considered as exceeding the grasp of reason. What then can the reason do exactly?