This title was first published in 2001. The encyclical "Fidel et Ratio", dealing with faith and reason and their specific catholocity, may well turn out to be the most important document of the modern Catholic Church on the subject announced in its title and on philosophy understood as a mirror of peoples' cultures. "Fidel et Ratio" wants to put in motion again faith that thinks (a different thing from the believer that thinks) and the issue of truth, thus opening up a higher dialogue with late modernity and postmodernity. Among the various stimuli proposed by the encyclical to start up again this belief that thinks, a significant one is the methodology of collaboration and circularity between reason and faith, and philosophy and theology, to their mutual advantage. The intention of the author is a questioning and reflecting on the nexus between philosophy and revelation without excluding a priori the insights of the above-mentioned encyclical.