“Philosophy is the knowledge of everything that exists as determined by their causes which are open to the investigation of reason alone.” As such, philosophy is encompassing of human knowledge, extending beyond sense experience to what may be known through the rational mind and inference. The first philosophers began with sense knowledge and sought to provide explanations for experiences gained through sense. The intelligibility of the world meant that they proceeded from mere experience to an intelligence about the world. From there they reasoned considering causes. This naturally led to the pursuit of ultimate causes, which itself opens the question of the nature of God.
Although human reason and philosophy alone can never plumb the depths of reality, they can bring man to a greater understanding of the universe and, by extension, its Creator. In this way, philosophy serves to complement theology. Theology is “logically reasoned discourse about God.” As such, it necessarily overlaps and may draw from philosophy, in which is found the summit of human reason on its own. As theology and philosophy are both expressive of reality, and God is Himself the source of truth in both, they cannot contradict one another. Rather, philosophy, reason, serves to explain truths of theology, of faith. It is not, of course, that God is in need of philosophy, but we are. Philosophy is finally a matter or reason, and theology builds upon reason. This seems to me to be why philosophy is called the handmaid of theology.
As Pope Leo XIII wrote, “it is in the very nature of man to follow the guide of reason in his actions.” Man is a rational being who relies always upon his reason, which suits him to consider his Creator and Final Cause.