“For it is owing to their wonder that men both now begin and at first began to philosophize.”
Classically (and etymologically), Philosophy is the “love of Wisdom.” “Love,” in this sense, is the eros or “passionate desire” for Wisdom and Truth. Philosophy investigates and seeks to answer the perennial, fundamental questions of human existence: “what is the purpose of life,” “what does it mean to live well,” “what is truth,” “can truth be known,” “what is good and evil,” etc. Given this breadth of inquiry, Philosophy qua philosophy is not an end in itself; instead Philosophy encompasses all disciplines / knowledge as they tend towards Wisdom and Truth.* Since all Wisdom and Truth are grounded in God, Philosophy naturally leads to Theology. Furthermore, because all Wisdom and Truth is essentially theological, Philosophy itself is always at the service of Theology; as Theology is the Queen of the Sciences, Philosophy is the ancilla theologiae (the Handmaiden of Theology). Philosophy serves as the ancilla theologiae in three ways:
Ratio (method / procedure) First, Philosophy equips Theology with the rules of thought / Reason by which the mind operates, the dialectic of argumentation by which the mind investigates Truth, and the analytic and synthetic processes by which the mind discovers Truth.
Auctoritas (authority) Second, Philosophy provides Theology with insights discovered by the great philosophers (both Christian and non-Christian) who arrived at the Truth with the light of Natural Reason and practices deference to their authority.
Concordia (union / harmony) Third, Philosophy coordinates its discovered Truths with Theological Doctrine and subordinates these Truths to Revelation.
*Related to this definition, philosophy is also the investigation into the presuppositions of any subject / discipline. It asks and attempts to answer the foundational questions of all areas of study (e.g., “how ought we to proceed in the study of X?”). Thus, philosophy incorporates the Seven Liberal Arts and the Four Sciences as it provides a “philosophy” of each of these disciplines: e.g., “the philosophy of grammar,” “the philosophy of science,” “the philosophy of arithmetic,” etc.