St. Thomas Aquinas’s Philosophy and the Existence of God
Demonstrating God’s Existence
St. Thomas Aquinas’s philosophy primarily aimed to demonstrate the existence of God. Rejecting a priori arguments like St. Anselm’s ontological argument, Aquinas believed knowledge originates in experience. He employed a posteriori arguments based on sensory experience and demonstration quia, which allows detecting something’s existence without prior knowledge of its nature. This led him to formulate the Five Ways:
- First Way (Cosmological Argument): Inspired by Aristotle, this argument reasons from the world’s movement to a First Unmoved Mover. As there cannot be an infinite regress of movers, there must be a First Mover, which is God.
- Second Way (Efficient Causality): Also inspired by Aristotle, this argument proceeds from subordinate causes to a First Uncaused Cause. Since a cause cannot cause itself, and an infinite chain of causes is impossible, there must be a First Uncaused Cause, which is God.
- Third Way (Contingency): Based on Maimonides, this argument reasons from the world’s contingency to a Necessary Being. Since contingent beings depend on other beings for their existence, there must be a Necessary Being who does not receive existence from another, which is God.
- Fourth Way (Degrees of Perfection): Inspired by Neoplatonists, this argument reasons from degrees of perfection to an Infinitely Perfect Being. Since we experience degrees of perfection, and an infinite chain of increasing perfection is impossible, there must be a maximum degree of perfection, which is God.
- Fifth Way (Cosmic Order): This argument reasons from the world’s order to an Ordering Intelligence. Since beings act with purpose, there must be an intelligence guiding them. Without an infinite chain of ordering intelligences, there must be a Supreme Intelligence, which is God.
God’s Nature and Relation to Creation
Having demonstrated God’s existence, Aquinas explored God’s distinction from the rest of reality. He adopted the principle of composition, where beings (except God) are composed of essence and existence. Essence is the what of a being, while existence is the act of being. Existence unfolds at different levels of perfection, corresponding to the essences it actualizes. This leads to an ontological hierarchy with God, the most perfect being, followed by angels, humans, and other things. Only in God are essence and existence identical, encompassing all possible perfections. Aquinas used Plato’s notion of participation to explain how created beings partake in existence to varying degrees, depending on their essences.
Knowing God
To understand God’s essence, Aquinas employed analogy, comparing God to humans in two ways:
- Negative Way: Denying in God what is negative in humans.
- Positive Way (Eminence): Considering God perfect in all qualities found in humans.
Theory of Knowledge
Aquinas’s theory of knowledge, based on Aristotle, posits that human knowledge begins with the senses. Knowledge involves abstracting from sensory experience to universal concepts. Sensitivity captures perceptible qualities, creating representations or “phantasms”. The intellect acts on these phantasms, stripping away individual elements, enabling the passive intellect to form universal concepts.
Faith and Reason
Aquinas believed reason and faith are distinct sources of knowledge, leading to philosophy and theology. Reason assists faith with scientific and dialectical methods, while faith guides reason by revealing Christianity as the only truth. In case of conflict, Aquinas asserted that the error lies with reason.
Natural Law and Happiness
Aquinas stated that happiness is humanity’s ultimate end, achieved by fulfilling natural law. Natural law is the spontaneous emanation of God’s eternal law within humans, known through our natural inclinations. Eternal law is the essential and primary sanction of all other laws. Aquinas identified three types of human inclinations:
- As a substance (self-preservation).
- As an animal (procreation).
- As rational (pursuit of truth and social life).
The primary precept of natural law is “good is to be done and pursued, and evil is to be avoided.” Aquinas added that for social life, humans must establish positive laws derived from natural law.