Metaphysics
Act and potency
Change is intelligible because a being can move from capacity to actuality.
Potency names a real capacity; act names fulfillment or determination. The distinction explains motion, development, limitation, and the difference between finite creatures and God.
- Potency is not sheer nothing but an ordered capacity for perfection.
- Pure act names God because no unrealized potency remains in Him.
- Many Thomist proofs depend on this distinction before they name it explicitly.
Related
essence and existence
Metaphysics
Essence and existence
What a thing is and that it is are distinct in creatures.
Essence answers the question of nature or quiddity; existence answers whether the thing is actual. Thomism treats created beings as receivers of existence rather than self-explanatory realities.
- This distinction grounds participation and creaturely dependence.
- It helps explain why contingent beings do not account for themselves.
- The distinction becomes central in natural theology and metaphysics of creation.
Metaphysics
Substance and accident
A substance exists in itself; an accident exists in another.
The distinction protects the difference between what a thing fundamentally is and the changing features that inhere in it. It lets Thomism speak precisely about persistence through change.
- Color, quantity, and relation are classic accidental modes.
- The substance is not a hidden object behind appearances but the being that underlies them.
- This distinction matters in both ordinary ontology and sacramental theology.
Nature
Form and matter
Material substances are composed of what is determined and what can be determined.
Matter is the principle of potentiality in bodily things; form gives actuality, intelligibility, and kind. Together they explain generation, corruption, and embodied unity.
- Form is not merely shape but the principle that makes a thing the kind of thing it is.
- Matter helps explain individuation and change in corporeal beings.
- The soul is treated as the substantial form of the living body.
Anthropology
Nature
Nature names an intrinsic principle of operation ordered to characteristic ends.
To ask about a nature is to ask what sort of being something is and how it is apt to act. Thomist ethics and politics depend on this account of stable human powers and goods.
- Nature is not opposed to reason; human nature includes rationality.
- Natural inclinations are clues to goods, not brute impulses to obey blindly.
- Grace presupposes nature because it perfects a real subject with real powers.
Ethics
Habit
A habit is a stable quality that disposes a power to act well or badly.
Habits explain why repeated action changes the agent. They make action easier, quicker, and more connatural, which is why Thomism treats moral formation as more than isolated choices.
- Virtues and vices are species of habits.
- Habit belongs to powers such as intellect and will, not only to external routine.
- Without habits, moral growth would have no stable interior form.
Ethics
Virtue
Virtue perfects a power for good action according to reason and, ultimately, grace.
For Aquinas, virtue is not mere rule-following. It is an excellence that orders appetites, judgment, and action toward genuine flourishing and prepares the person for higher ends.
- Moral virtues shape appetite; intellectual virtues perfect understanding and judgment.
- Prudence is central because right action requires right reasoning about means.
- The theological virtues order the person beyond merely natural happiness.
Ethics
Law
Law is an ordinance of reason for the common good, made by legitimate authority, and promulgated.
This definition shows why Thomist political thought is neither pure voluntarism nor private intuition. Law is rational, public, and ordered to shared human flourishing.
- Natural law expresses the rational creature’s participation in eternal law.
- Human law applies general principles to concrete communities.
- An unjust law can fail in its full moral character as law.
Theology
Grace
Grace is a gratuitous divine gift that heals and elevates the creature.
Thomism does not treat grace as a rival to nature. Grace presupposes a created subject, perfects its powers, and raises the person toward a supernatural end that nature alone cannot attain.
- Sanctifying grace establishes a new mode of life in the soul.
- Grace does not cancel freedom but enables freer action toward the highest good.
- The relation of nature and grace is one of elevation, not replacement.
Related
virtue and beatitude
Theology
Participation
Creatures possess limited perfections by receiving them, not by being their source.
Participation helps Thomism speak about likeness and dependence at once. A creature is truly good, wise, or existent, but only by sharing in perfections that belong to God in a higher mode.
- Participation guards against both pantheism and radical separation.
- It explains why created goodness is real yet finite.
- The notion supports analogy, causality, and the doctrine of creation.