Comparing Hume, Kant, Aquinas, and Descartes

Hume and Kant: Philosophical Similarities

David Hume and Immanuel Kant share several fundamental perspectives:

• The Enlightenment Ideal: Both philosophers share the Enlightenment ideal of using reason autonomously and applying it to all ambits of human life.
• Limits of Theoretical Reason: For both Hume and Kant, theoretical reason cannot penetrate or conceive of realities that are beyond experience. Hume discovered that reason operates within the limits of sensible experience, while Kant maintained that pure a priori concepts can apply only to phenomena.
• Agnosticism and Metaphysics: Both are agnostics regarding metaphysics and declare the impossibility of metaphysics as a science.
• Practical Application: Both share a concern for the themes of reason applied to political, ethical, and religious matters.

Key Differences Between Hume and Kant

• A Priori Concepts: Kant admits, unlike Hume, the validity of a priori concepts (such as the concept of substance) that do not originate from experience. Hume admits only ideas that correspond directly to a preceding impression.
• Skepticism vs. Science: Kant rejects the threat of skepticism found in Hume's philosophy and admits the faktum of the unquestionable validity of science. Scientific judgments are universal and necessary for Kant, whereas Hume admits only highly probable scientific judgments, not necessary or universal ones.
• Ethics and Moral Theory: In Kant's moral theory, the moral law is grounded in reason, which is incompatible with modern emotional theories of ethics. In Hume, ethics is not based on reason but on sentiment—the sentiments of approval or disapproval. In Kant, practical reason determines duty and responds to the question of how one should act; sentiments are incapable of providing a general principle of morality. Good or bad in ethics does not depend on a sentiment of approval or disapproval, but on the compliance of the good will with duty for duty's sake.

St. Thomas Aquinas and Descartes: Shared Views

• Reason and God: Both admit that human reason can prove the existence of God.
• Principle of Causality: Both Aquinas and Descartes rely on the principle of causality, although they apply it with different characters.
• God as the Ultimate Explanation: Both use God ultimately to explain movement—St. Thomas Aquinas for the movement of people (the Prime Mover) and Descartes for the movement of the world.

Contrasting Aquinas and Descartes

• Reason and Faith: St. Thomas Aquinas seeks to prove the "Five Ways" to support faith with reason. For Descartes, reason is autonomous and is not subject to faith.
• Starting Points: The proofs of St. Thomas Aquinas are based on real objects found through experience and interpreted metaphysically. Descartes starts from the innate idea of a perfect or infinite being. Thus, Descartes' starting point is ideal, while St. Thomas Aquinas' is real.
• Existence vs. Ideas: Aquinas wants to explain the existence of facts. Descartes applies his reasoning to the idea of a perfect or infinite being.
• Attributes of God: For Aquinas, God is necessary; for Descartes, the primary attribute of God is infinity.