Thomas Aquinas

Justice

Justice is the constant will to render to each person what is due. Natural law grounds it.

Good and Evil

Every agent acts for the good. Evil is the failure of a good: a defect in being or action.

Virtue and Vice

The four cardinal virtues are natural; faith, hope, and charity are infused by grace.

Happiness

Perfect happiness is the beatific vision; imperfect happiness is virtuous activity in this life.

Love

Charity is friendship with God: love elevated by grace to share in the divine life.

Courage

Fortitude is a cardinal virtue that enables the soul to endure dangers and hardships in pursuit of the good, perfected by the supernatural gift of martyrdom.

Desire

Desire is the rational appetite's movement toward the good apprehended by the intellect; the will naturally desires God as its final end.

Punishment

Punishment serves justice first and reform second: the penalty restores the order that sin has broken, and may also heal the sinner.

Duty

Natural law commands duty through reason; justice alone among virtues implies obligation, but all virtues fall under natural law's precepts.

Emotion

The passions are movements of the sensitive appetite; eleven passions ordered under the concupiscible and irascible powers.

Pleasure and Pain

Joy and sorrow are movements of the appetite in satisfaction or frustration; pleasure may attend virtue but is never its end.

Honor

Honor is owed in justice to genuine excellence, but its supreme object is God — vainglory distorts by seeking it for oneself.

Habit

Habits are qualities that perfect or diminish the faculties they modify — and supernatural virtues require infusion, not just repetition.

Temperance

Temperance moderates the appetite for bodily pleasures — the creation is good, and ordered enjoyment is virtue, not sin.

Custom and Convention

Custom has the force of a law, abolishes law, and is the interpreter of law; the habitual conduct of a community expresses its reason in public form.

Liberty

Free will follows from reason. Liberty is fulfilled when we choose rightly under natural law.

Government

Legitimate government derives from natural law. Tyrannical rule forfeits its authority.

Law

Four kinds of law (eternal, natural, divine, human) form a single hierarchical order.

War and Peace

War can be just, but only under strict conditions. Peace is the work of justice.

Democracy

The best regime is mixed: monarchy tempered by aristocratic and democratic elements, with laws made by or for the people.

State

The state is a natural institution ordained by God for the common good, governed by law that participates in eternal reason.

Wealth

Wealth is a legitimate but subordinate good; private property is justified by natural law yet bound by the claims of justice and charity.

Revolution

Resistance to a tyrant is not sedition; the tyrant himself is the seditious one.

Tyranny

Tyrannical government is altogether corrupt; the tyrant who oppresses his subjects is himself guilty of sedition.

Family

Marriage is both a natural institution and a sacrament — its indissolubility follows from both levels, and children are persons entrusted, not possessions owned.

Labor

Labor is both a natural obligation and a penitential discipline: it is owed to the community, sanctified by its connection to the divine order, and made dignified by craft.

Slavery

Slavery is against the first intention of nature but may be permitted as a consequence of sin and for the common good.

Constitution

The best constitution is a mixed regime in which law expresses the reason of the community, not the will of the prince alone.

Monarchy

Monarchy is the best form of government because unity of rule mirrors the unity of divine governance over creation.

Aristocracy

The best regime is mixed, combining monarchy, aristocracy, and democracy so that virtue governs and all have a stake.

Oligarchy

Oligarchy is a corrupt form of government in which the few rule for their own enrichment rather than for the common good.

God

God's existence can be proven five ways. His essence is being itself: ipsum esse subsistens.

Soul

The soul is the form of the body yet subsists after death: a rational substance awaiting resurrection.

Being

God is the sheer act of being, *ipsum esse subsistens*, and every creature is a composition of essence and existence.

Cause

Every finite cause points beyond itself; the chain of causes must terminate in a First Cause, God.

Time

Eternity is the simultaneous whole of interminable life; time is the measure of changing beings, aevum between them.

Religion

Faith is an intellectual virtue directed by the will; reason demonstrates God's existence but cannot penetrate the mysteries of revelation.

Immortality

The soul, as subsistent form, survives death but remains incomplete without the body; resurrection fulfills a natural desire.

Sin

Sin is an act contrary to the eternal law: mortal sin destroys charity and turns the soul entirely from its last end, while venial sin disorders the soul without severing it from God.

Theology

Natural theology proves God by reason; sacred theology, grounded in revelation, is a true science whose principles come from God's own knowledge.

Prophecy

Prophecy is a supernatural light by which God elevates the prophet's intellect to know what exceeds natural reason.

Metaphysics

Metaphysics studies being as being; sacred theology studies God through revelation; the two sciences complement each other without conflict.

Chance

Chance is real relative to created causes, but all coincidences are foreseen and ordered by divine providence.

Change

Change is the reduction of potency to act, but creation itself is not a change, for it presupposes no subject.

Form

Form is the act by which matter is determined to be this kind of thing; in each substance there is only one substantial form.

Matter

Prime matter is pure potency, created together with form by God, and it is what makes one body numerically distinct from another.

Necessity and Contingency

Contingent beings require a necessary being; God wills some effects necessarily and some contingently.

One and Many

Unity is a transcendental (convertible with being) and God alone is absolutely simple.

Space

Place belongs to bodies; angels are in place by application of power, not by being contained.

World

The world is created by God from nothing but could, for all reason shows, have existed eternally; that it began in time is known only by faith.

Opposition

Evil is not a positive opposite of good but its privation, a lack in what ought to be present.

Relation

In God, the persons are subsistent relations: the Father is nothing other than the relation of paternity, and this relation is identical with the divine essence.

Same and Other

In the Trinity, the persons are relationally distinct yet numerically identical in essence; real distinction without any division of substance.

Quality

Quality perfects substance in its manner of being; habits and dispositions are qualities of the soul that order it toward its proper operation.

Quantity

Quantity is the first accident of corporeal substance, the property that follows directly upon matter and makes a body divisible into parts.

Angel

Each angel is a distinct species: a subsisting form, incorporeal, knowing by infused ideas without discourse.

Eternity

Eternity is the simultaneous-whole possession of interminable life, proper to God alone, with aeviternity as a middle term for angels.

Fate

Fate is the order of second causes through which providence works; real, but subordinate to God and compatible with freedom.

Infinity

God alone is absolutely infinite, because in God essence and existence are identical; all created infinities are relative and negative.

Truth

Truth is the adequation of intellect to thing. Being is primarily true; statements are true derivatively.

Knowledge

All knowledge begins in the senses. The intellect abstracts universal forms from particular things.

Mind

The intellect abstracts universals from sensible things: mind is the soul's highest power, oriented to being.

Experience

Nothing is in the intellect that was not first in the senses, yet the agent intellect actively abstracts universals from sense experience.

Will

The will is rational appetite, necessarily drawn to happiness but free in its choice of particular goods.

Language

Human language signifies through concepts; we can name God only by analogy, never univocally.

Memory and Imagination

Memory and imagination are interior senses requiring bodily organs; the estimative power recognizes the useful and harmful by instinct.

Rhetoric

Sacred rhetoric must combine truth with persuasion; the preacher's moral character is itself an argument.

Idea

Ideas are formed by the intellect's abstraction from sense experience, though their eternal archetypes exist in the mind of God.

Judgment

Truth resides in the judgment of the intellect as it conforms to the thing known.

Sense

Sensation is a spiritual immutation of the organ by the sensible form; it bridges the bodily and the intellectual.

Sign and Symbol

Words signify things through the mediation of concepts; the names of God are analogical, neither univocal nor equivocal.

Hypothesis

The articles of faith serve theology as hypotheses serve a science: they are the unproved starting points from which all reasoning proceeds.

Mathematics

Mathematics abstracts quantity from sensible matter, yielding certainty because its objects are stable and determinately known.

Dialectic

Dialectic and disputation serve theology by clarifying objections and sharpening the statement of truth, though they cannot prove articles of faith.

Principle

First principles of reason are self-evident; first principles of morals are grasped by synderesis as naturally as axioms are grasped by intellect.

Reasoning

Human reasoning is discursive, moving step by step from principles to conclusions; angels and God know by direct intuition.

Logic

Logic is the liberal art that directs the act of reason itself, indispensable to every science as the instrument that forms the mind before it engages its subject matter.

Definition

Real definitions express the essence of things; but for immaterial beings like God, strict definition fails and we must resort to analogy and negation.

Universal and Particular

Universals exist in three ways: before things in the divine mind, in things as their forms, and after things in the human intellect — each a legitimate perspective on the same reality.

Science

Sacred doctrine is a science; it borrows its first principles from a higher science — the knowledge of God — just as music borrows its principles from arithmetic.

Education

Teaching is real but dependent: the teacher arranges signs; the learner discovers truth with the natural light of intellect.

Nature

Nature is God's handiwork: created beings with their own principles, oriented toward ends.

Wisdom

Wisdom judges all things by their highest cause; it exists as philosophical virtue, theological virtue, and supernatural gift.

Prudence

Prudence is right reason applied to action — the charioteer of the virtues, with eight integral parts that constitute excellent practical reasoning.

Philosophy

Philosophy and sacred theology are distinct sciences with distinct methods, yet reason and revelation cannot contradict each other.

Man

Man is the union of rational soul and body, occupying the boundary between the material and the spiritual orders.

Beauty

Beauty is what pleases when seen: it requires integrity, proportion, and clarity.

Art

Art is right reason in making: an intellectual virtue concerned with the goodness of the work, not the worker.

Life and Death

The soul as the form of the body naturally survives death; the resurrection of the body completes what philosophy can only partially explain.

Animal

Animals possess sensitive souls but lack reason and free will; they act by instinct, not deliberation.

Element

The four elements are the simplest natural bodies, possessing substantial forms that ground their powers.

Medicine

Medicine is a subordinate art that cooperates with nature's healing power, and the habits of the body mirror the habits of the soul.

Astronomy

The celestial bodies declare the glory of God, but astronomical knowledge is not necessary for salvation; faith rests on revelation, not the stars.

Evolution

Species are fixed because each is defined by its substantial form, which cannot gradually shade into another; creation is complete.

Physics

Physics studies mobile being as such; it is a genuine science subordinate to metaphysics, and its principles are derived from the philosophy of nature.