Thomism is the system of philosophy developed by Thomas Aquinas, a Catholic scholar. Aquinas harmonized the philosophy of Aristotle with Christian theology, creating a system that became among the most influential in history. Aquinas’ philosophy was popular during his lifetime. After his death, a small minority of Catholic leaders condemned his teachings as heresy. Their efforts reduced the popularity of his work, but only temporarily.
In response to the Reformation, the Catholic Church heavily endorsed the work of Aquinas, including Thomism, elevating it to a status second only to the Bible itself. Other philosophical systems today disagree with Thomism on many points, but Thomism remains a dominant philosophical worldview.
Thomism is strongly grounded in reason, specifically in opposition to “blind faith.” It holds the laws of non-contradiction and causality as the fundamental principles of reality. According to Thomism, most of nature and theology can be apprehended through observation and reason. That which can be known by reason, accordingly, should be used to judge what is known only by faith. Thomism recognizes, however, that certain truths are only knowable by special revelation.
Thomism is also empiricist, meaning it teaches that observations and experiences are necessary for knowledge. It claims that we cannot argue for God’s existence on the basis of direct experience; we can only argue for God by interpreting what we see, feel, and understand. This philosophy rejects the Rationalist claim that pure logic or reasoning—without any observations or empirical data—can be used to draw reliable conclusions.
Thomism also approaches knowledge of God via “negative theology.” This is the belief that it is necessary for human beings to apprehend God through metaphors and analogies. Since God is unique, transcendent, and infinite, He is necessarily beyond our full comprehension. In order to at least partly comprehend God, we must use figurative or analogous terms relevant to our experiences. This approach also implies that portions of Scripture can be interpreted figuratively, depending on the context.
The correspondence theory of truth is a core part of Thomism. This is the idea that “truth” can be defined as conforming to some external, objective reality. Thomism supposes both empiricism and objective realism, which both claim our senses are useful and that the world can be understood more or less as it actually is. Thomism also teaches a fusion of the body and soul, which differs in many ways from classical dualism.
Thomism poses a distinction between “essence” and “existence.” It posits that God alone is absolute and all other things are finite and imperfect. Therefore, only God has an essence identical to His existence. He is the one and only pure expression of essence, substance, and existence. For all other things, their “what” (essence) is different from their “is” (existence). This also means that evil does not exist, in and of itself, except as a lack of “good.” Something is evil insofar as it violates its purpose, or the “cause” of its existence.
According to Thomism, all living things possess some type of soul, but humans alone have an immortal, “rational” soul. Our ability to use reason, per this philosophy, is a supernatural quality that other forms of life do not possess.
Thomism is also the source of Aquinas’ “Five Ways,” which are introductory means to argue for the existence of God. These are the first mover argument, the first cause argument, the contingency (necessity) argument, the ontological (perfection) argument, and the teleological (design) argument. These are often misunderstood, and assumed to be Aquinas’ best and strongest case for the Christian God. In truth, the “Five Ways” are merely meant to be basic principles to introduce the concept of Christian theism on rational grounds.