Western Philosophy: Core Concepts from Aquinas to Rousseau

Posted on Jun 15, 2025 in Philosophy and ethics

Thomas Aquinas: Key Philosophical Concepts

Reason and Faith

Reason and faith are distinct but complementary paths to truth. Theology, guided by faith, helps discern the truth of statements.

The Five Ways (Proofs for God’s Existence)

Aquinas proposed five arguments for the existence of God:

  • Motion: Everything in motion was put in motion by something else, leading to a First Mover.
  • Efficient Cause: Every effect has a cause, leading to a First Cause.
  • Contingency and Necessity: Contingent beings depend on a necessary being for their existence.
  • Degrees of Perfection: Gradations of perfection imply a supreme standard of perfection.
  • Teleological Argument (Governance of the World): The order and purpose in the world point to an intelligent designer.

Metaphysics: Essence and Existence

Aquinas distinguishes between Essence (what a thing is, its defined entity) and Existence (the fact that it is).

Theory of Knowledge

  • Sensible Knowledge: Humans perceive reality through the senses.
  • Abstract Knowledge: Involves two operations: intellectus agens (active intellect) and intellectus possibilis (passive intellect).

Problem of Universals

Aquinas’s position on universals:

  • Ante Rem: Universals exist in the divine mind (before creation).
  • In Re: Universals exist in things themselves (as forms or essences).
  • Post Rem: Universals exist as concepts in the human mind (after abstraction from particulars).

Ethics: Natural and Positive Law

  • Natural Law: Unwritten, universal, and unchanging moral principles inherent in human nature.
  • Positive Laws: Written laws enacted by human authority.

Politics and Governance

The State must promote the divine eternal law. Temporal power is subordinate to spiritual authority.

René Descartes: Rationalism and Metaphysics

The Cartesian Method

Descartes’ method for structuring reason:

  • Intuition: Grasping truths with absolute certainty.
  • Deduction: Deriving conclusions from intuitions.

Rules to guarantee certainty:

  • Evidence: Accepting only clear and distinct ideas.
  • Analysis: Breaking down complex problems into simpler parts.
  • Synthesis: Reconstructing from simple ideas to complex ones.
  • Enumeration: Reviewing all steps to ensure nothing is omitted.

Methodological Doubt

Systematic doubt of all knowledge derived from tradition, faith, or authority. It serves as a starting point to arrive at indubitable truth.

Metaphysics: The Three Substances

Descartes categorizes ideas as: adventitious (from experience), fictitious (imagined), and innate (born with us). The idea of an infinite, perfect being (God) is innate and can only originate from God himself.

He proves God’s existence through:

  • Argument from the Objectivity of Ideas: The idea of an infinite being requires an infinite cause.
  • Ontological Argument: God, as a perfect being, must necessarily exist.

He distinguishes three substances:

  • Res Cogitans: Thinking substance (mind/soul).
  • Res Infinita: Infinite substance (God), the source of all perfection.
  • Res Extensa: Extended substance (matter/bodies), constituting the physical world.

David Hume: Empiricism and Skepticism

Elements of Knowledge

  • Impressions: Vivid, immediate perceptions from the senses.
  • Ideas: Faint copies of impressions, formed in the mind.

Laws of Association of Ideas:

  • Resemblance
  • Contiguity (in space or time)
  • Causation

Types of Knowledge

  • Relations of Ideas: Necessary truths (e.g., mathematics, logic), discoverable by reason alone.
  • Matters of Fact: Contingent truths about the world, based on experience and probability.

Critique of Metaphysical Concepts

  • External Reality: We only know impressions; we cannot prove the existence of external objects beyond our perceptions.
  • God: The concept of God is not derived from any impression, making its existence unknowable through experience.
  • Self (I): The self is not a permanent, unified impression but a bundle of constantly changing perceptions.

Phenomenalism and Skepticism

  • Phenomenalism: We only know phenomena (appearances/impressions), not the underlying reality.
  • Skepticism: True knowledge about matters of fact is not possible through experience, only probability.

Moral Emotivism

Hume criticizes moral rationalism, proposing:

  • Emotivism: Moral judgments are based on feelings and sentiments, not reason.
  • Utilitarian Tendency: Moral approval arises from actions that promote social utility and well-being.

Religion: Agnosticism

The existence of God is an inaccessible mystery, leading to an agnostic stance.

Jean-Jacques Rousseau: Society and the Social Contract

The State of Nature

Humans are naturally good and happy in the state of nature, driven by self-love (amour de soi) and compassion (pitié). However, society corrupts them, leading to inequality and moral degeneration, primarily due to:

  • Private Property: The root cause of economic inequalities.
  • Arbitrary Power: Leading to despotism and loss of freedom.

The Social Contract

A social contract is necessary to establish an egalitarian society of free citizens.

The General Will

The collective will of citizens, formed through the social contract, aiming for the common good. It is expressed through laws and political actions that serve the Common Interest.