Thomas Aquinas: Core Philosophical and Theological Concepts

Posted on Mar 31, 2025 in Philosophy and ethics

Theology and Philosophy: Faith and Reason

Contrary to the theory advocated by some Averroists, Thomas Aquinas (TA) posits that there are truths specific to faith (theology), others specific to reason (philosophy), and some shared by both (preambles of faith, such as the existence of God). There can be no contradiction between these two forms of wisdom. In the event of an apparent conflict, faith must prevail as it originates from God’s revelation. Therefore, philosophy is considered the handmaid of theology.

Proving God’s Existence: The Five Ways

Regarding the existence of God, Thomas Aquinas raises the following questions:

  • Is God’s existence self-evident? No.
  • Can it be proven? Yes.
  • How? Proof can be either a priori (from cause to effect) or a posteriori (from effect to cause).

Aquinas argues that God’s existence cannot be proven a priori but can be proven a posteriori. This means God’s existence can be demonstrated starting from observable effects in the world to arrive at their ultimate cause.

TA demonstrates God’s existence through his Five Ways (Quinque Viae), which share a common structure:

  1. Starting point: Observation of phenomena in the world.
  2. Principle of causality: Everything that changes or exists contingently has a cause.
  3. Impossibility of infinite regress: The chain of causes cannot go back infinitely; it must terminate in a first cause or source.
  4. Conclusion: God exists as the ultimate cause/source.

The Five Ways are:

  1. The Argument from Motion concludes with an Unmoved Mover (God).
  2. The Argument from Efficient Cause concludes with a First Cause (uncaused).
  3. The Argument from Contingency concludes with a Necessary Being.
  4. The Argument from Degrees of Perfection concludes with a Supremely Perfect Being.
  5. The Argument from Design (Teleological Argument) concludes with an Intelligent Designer.

While we cannot fully comprehend God’s essence in this life, we can know that He exists and attribute perfections like justice, omniscience, etc., to Him through analogy.

The Nature of the World and Created Beings

There are two fundamental types of beings:

  • God: The Supreme Necessary Being, whose essence is His existence.
  • Created Beings: All other beings, which are contingent (their existence is not necessary). They are composites, characterized by distinctions such as:
    • Matter and Form
    • Essence and Existence
    • Potency and Act

These concepts incorporate and build upon Aristotelian metaphysics.

The Human Being: A Union of Body and Soul

The human being is a composite of body and soul. Similar to Aristotle, Aquinas views this as a substantial union, meaning the soul is the form of the body. However, unlike Aristotle, Aquinas holds that the human soul is spiritual and immortal; it survives the death of the body. Following Aristotle (the Peripatetic), Aquinas distinguishes types of souls:

  • Vegetative soul: Found in plants (functions of growth, nutrition, reproduction).
  • Sensitive soul: Found in animals (includes vegetative functions plus sensation and locomotion).
  • Rational soul: Found in humans (includes vegetative and sensitive functions plus intellect and will).

Crucially, the human being is a person, defined by Boethius as ‘an individual substance of a rational nature’.

Aquinas’s Theory of Knowledge

Similar to Aristotle, Aquinas believes knowledge begins with sensory experience. The process of acquiring intellectual knowledge involves several stages:

  1. The senses perceive an external object.
  2. An image, or phantasm, of the particular object is formed in the imagination.
  3. The agent intellect abstracts the universal form (the essence or concept) from the particularity of the phantasm.
  4. The possible intellect (or patient intellect) receives this universal form, thereby understanding the object’s essence.
  5. The intellect can then relate this universal understanding back to the phantasm to recognize the individual object from which it was derived (conversio ad phantasmata).

Thomistic Ethics: Happiness and Divine Law

Thomistic ethics is eudaimonistic, centered on achieving happiness or flourishing (*beatitudo*). Like Aristotle, Aquinas states that all humans naturally seek happiness. However, Aquinas distinguishes between imperfect happiness (achievable to a degree in this life through virtue) and perfect happiness. Perfect, ultimate happiness consists in the beatific vision – the direct contemplation of God’s essence in the afterlife.

To achieve this ultimate end, humans must practice virtue (both moral and intellectual) and follow God’s law. Aquinas outlines a hierarchy of law:

  • Eternal Law: God’s rational plan and governance for all creation, existing in the divine mind.
  • Natural Law: Humanity’s participation in the eternal law through reason. It directs us to do good and avoid evil, based on our natural inclinations.
  • Divine Law: Law revealed by God through Scripture and tradition (e.g., the Ten Commandments, the teachings of Christ). It guides humans towards their supernatural end (the beatific vision).
  • Human Law: Specific laws created by human societies to regulate communal life. To be just, human laws must be derived from and conform to the natural law.

Aquinas on Politics and Just Government

Following Aristotle, Aquinas believes that humans are naturally social and political animals. Society is necessary for human flourishing. The common good of the political community is a primary goal, taking precedence over private interests, although the inherent dignity of each individual must be respected.

Just human laws must be ordered to the common good and derived from the natural law. Aquinas analyzes forms of government, distinguishing between good forms aimed at the common good and their corrupt counterparts focused on the ruler’s private interest:

  • Good: Monarchy (rule by one), Aristocracy (rule by the few best), Polity/Republic (rule by the many).
  • Corrupt: Tyranny (perversion of monarchy), Oligarchy (perversion of aristocracy), Democracy/Mob Rule (perversion of polity).

Tyranny is considered the worst form of government. While Aquinas often expressed a preference for monarchy (arguing it best reflects the unity of God’s rule over the universe and promotes social unity), he advocated for a system with checks and balances – ideally a mixed government combining elements of monarchy, aristocracy, and polity, or at least a monarchy limited by law and custom, not absolute rule.