Classical Western Philosophy II: Medieval Philosophy and the Rationalists (Descartes, Spinoza, Leibniz)

This chapter takes the medieval block of Unit 2, Augustine on evil, Anselm's ontological argument and Aquinas on faith and reason, essence and existence and the existence of God, and then the three continental rationalists. Descartes supplies method, doubt, the cogito, innate ideas, dualism, proofs of God and interactionism; Spinoza one substance, God or Nature, parallelism, pantheism, three orders of knowing and the intellectual love of God; Leibniz the monads, truths of reason and fact, innate ideas, proofs of God, the three great principles, pre-established harmony and freedom. The rationalists are linked by the problem Descartes left: how two substances as unlike as mind and body can be related. The syllabus spells the third name "Leibnitz", an older spelling still found in Indian textbooks; the chapter uses the standard Leibniz.

1. Medieval philosophy: Augustine, Anselm and Aquinas

St. Augustine (354–430) answered the problem of evil, how evil can exist if God is wholly good and all-powerful, in two moves. First, evil is not a substance created by God but a privation of good (privatio boni), as blindness is the absence of sight; this rejected the Manichaean dualism of two eternal principles that Augustine had once held. Second, moral evil arises from the misuse of free will by rational creatures (On Free Choice of the Will), and God permits it because a world with free creatures is better than one without them. Natural evil he traced to the Fall and to the goodness of the whole order, in which the parts may seem defective.

St. Anselm (1033–1109) gave the ontological argument in Proslogion 2. God is "that than which nothing greater can be conceived". Even the fool who says in his heart there is no God understands this phrase, so God exists in the understanding. But what exists in reality as well as in the understanding is greater than what exists in the understanding alone. So if God existed only in the understanding, something greater could be conceived, which is contradictory; therefore God exists in reality. Proslogion 3 adds that God cannot even be conceived not to exist. The monk Gaunilo replied (On Behalf of the Fool) that the same reasoning would prove a perfect lost island; Aquinas rejected the argument because we do not know God's essence; and Kant later objected that existence is not a real predicate.

St. Thomas Aquinas (1225–1274) held that faith and reason are two sources of truth that cannot contradict each other, since both come from God. Some truths about God, the "preambles of faith" such as his existence and unity, can be proved by reason; others, the "articles of faith" such as the Trinity and the Incarnation, are known only by revelation, though reason can show they are not impossible. In On Being and Essence he distinguished essence (what a thing is) from existence (that it is): in every creature the two are really distinct, since knowing what a phoenix is does not tell us whether one exists; in God alone essence is existence, and God is subsistent being itself, the pure act of existing.

Aquinas's Five Ways (Summa Theologiae I, q. 2, a. 3)
WayStarting pointConclusion
1. MotionThings are moved from potency to act by anotherA first unmoved mover
2. Efficient causationNothing is the efficient cause of itself; no infinite seriesA first efficient cause
3. ContingencyThings can be and not beA necessary being
4. GradationThings are more and less good, true, nobleA maximum that causes these perfections
5. GovernanceNon-intelligent things act for an endAn intelligent being directing nature

2. Descartes: method, doubt, cogito, innate ideas, God and dualism

René Descartes (1596–1650) wanted philosophy to have the certainty of mathematics. The Discourse on Method (1637) gives four rules: accept nothing as true that is not clearly and distinctly perceived; divide each difficulty into as many parts as possible; proceed in order from the simplest to the most complex; and make enumerations so complete that nothing is omitted. His criterion of truth is clarity and distinctness, which the Meditations (1641) finally guarantee through God's veracity: God is no deceiver, so what I perceive clearly and distinctly is true.

  • Methodological scepticism (Meditation I): doubt is a method, not a conclusion. The senses sometimes deceive; I cannot be sure I am not dreaming; and an all-powerful evil demon (malin génie) might deceive me even about arithmetic. Whatever survives such doubt is certain.
  • Cogito ergo sum: "I think, therefore I am" (Discourse IV; in Meditation II, "I am, I exist" is necessarily true whenever I put it forward or conceive it). Even to doubt or be deceived I must think, and to think I must exist. What I am is a thinking thing (res cogitans).
  • Three kinds of idea (Meditation III): adventitious (coming from outside, the sun as seen), factitious (made by me, a siren or hippogriff) and innate (implanted by God: the ideas of God, self, substance and mathematical truths).
  • Proofs of God: the "trademark" argument (Meditation III: my idea of an infinite, perfect being must have a cause with at least as much reality as the idea represents, and only God has it); the causal argument from my own existence; and the ontological argument (Meditation V: existence belongs to the essence of a supremely perfect being as three angles equal to two right angles belong to a triangle).
  • Cartesian dualism and interactionism: mind is thinking and unextended, body is extended and unthinking, and the two are really distinct substances. Yet they interact: the body causes sensations, the will moves the body, and Descartes located the meeting point in the pineal gland. Princess Elisabeth of Bohemia pressed the obvious objection: how can an unextended thing move an extended one?

3. Spinoza: one substance, God or Nature

Baruch Spinoza (1632–1677) set out his Ethics (published 1677) "in geometrical order", with definitions, axioms and propositions. Substance is "what is in itself and is conceived through itself". Since two substances could not share an attribute and a substance cannot be produced by another, there is only one substance, which is infinite, necessary and self-caused: God or Nature (Deus sive Natura). Substance has infinitely many attributes, of which we know two, thought and extension; modes are the particular affections of substance, finite things such as bodies and ideas. God as active, self-causing substance is natura naturans; the world of modes that follows from him is natura naturata.

  • Parallelism: "the order and connection of ideas is the same as the order and connection of things" (Ethics II, proposition 7). Mind and body are one and the same thing conceived under two attributes, so there is no interaction and no problem of interaction.
  • Pantheism and determinism: God is not a transcendent creator with purposes but the immanent cause of all things; everything follows from God's nature with necessity, and there is no contingency. Human freedom is not free will but understanding the necessity by which we act.
  • Three orders of knowing (Ethics II, proposition 40, scholium 2): opinion or imagination (from the senses and from signs, confused and inadequate), reason (adequate ideas of the common properties of things) and intuitive knowledge (scientia intuitiva), which grasps particular things as they follow from God's essence.
  • The intellectual love of God (amor Dei intellectualis) arises from the third kind of knowledge: joy accompanied by the idea of God as its cause. It is the mind's highest blessedness, and Spinoza says it is part of the infinite love with which God loves himself.

4. Leibniz: monads, two kinds of truth and pre-established harmony

G. W. Leibniz (1646–1716) held in the Monadology (1714) that the ultimate constituents of reality are monads: simple, unextended, indivisible substances with no parts. Their only internal states are perceptions (representations of the whole universe from a point of view) and appetitions (tendencies from one perception to the next). They are "windowless": nothing enters or leaves them, and each unfolds its states from within. Monads form a hierarchy: bare monads with unconscious perceptions, souls with memory, and spirits or minds with reason and apperception; God is the supreme monad.

Truths of reason and truths of fact
PointTruths of reasonTruths of fact
ModalityNecessary: the opposite is impossibleContingent: the opposite is possible
Governing principlePrinciple of (non-)contradictionPrinciple of sufficient reason
ExamplesMathematics, logic, "a triangle has three angles""Caesar crossed the Rubicon"
AnalysisFinite: reduces to identitiesInfinite: only God can complete it
  • Innateness of ideas (New Essays on Human Understanding, a reply to Locke): the mind is not a blank tablet but like a block of marble with veins that mark out the shape of Hercules. Necessary truths and ideas such as being, substance and identity are innate as dispositions, brought to awareness by experience but not derived from it.
  • Proofs of God: the ontological argument, completed by showing that the idea of a most perfect being is possible (consistent); the cosmological argument from sufficient reason (the series of contingent things needs a reason outside itself); the argument from eternal truths (they need a mind in which to exist); and the argument from pre-established harmony.
  • Three principles: non-contradiction (governs truths of reason), sufficient reason (nothing is so without a reason why it is so and not otherwise) and the identity of indiscernibles (no two substances are exactly alike; if they shared every property they would be one).
  • Pre-established harmony: since monads are windowless, mind and body do not interact; God set them in agreement at creation, like two clocks made so perfectly that they always keep the same time.
  • Problem of freedom: if every truth about me is contained in my complete concept, am I free? Leibniz answers that my actions are contingent (their opposite is not contradictory), spontaneous (they come from within) and intelligent (chosen by reason); reasons "incline without necessitating". His Theodicy (1710) argues that God chose the best of all possible worlds.
🧠 Three answers to the mind-body problem
Descartes: interactionism (the pineal gland). Spinoza: parallelism (one substance, two attributes). Leibniz: pre-established harmony (two clocks). Malebranche, though not in the syllabus, completes the set with occasionalism: God is the only true cause and acts on the occasion of mental or bodily events.

Key takeaways

  • Augustine: evil as privation of good; moral evil from free will. Anselm: "that than which nothing greater can be conceived" (Proslogion 2), with Gaunilo's island objection. Aquinas: faith and reason cannot conflict; essence and existence distinct in creatures, identical in God; the Five Ways.
  • Descartes: four rules of method; clarity and distinctness; doubt from senses, dreams and the evil demon; cogito; adventitious, factitious and innate ideas; trademark, causal and ontological proofs; dualism with interaction at the pineal gland.
  • Spinoza: one substance (Deus sive Natura), infinite attributes of which we know thought and extension, modes; parallelism (Ethics II P7); pantheism and determinism; opinion, reason and intuitive knowledge; amor Dei intellectualis.
  • Leibniz: windowless monads; truths of reason (necessary, contradiction) and of fact (contingent, sufficient reason); veined marble; identity of indiscernibles; pre-established harmony; freedom as contingency, spontaneity and intelligence.
  • Mind-body: interactionism (Descartes), parallelism (Spinoza), pre-established harmony (Leibniz), occasionalism (Malebranche).

Practice questions (10)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

  1. Who defined God as "that than which nothing greater can be conceived"?

    1. Anselm
    2. Gaunilo
    3. Augustine
    4. Aquinas
    Show answer

    Answer: A — Anselm

    Anselm's Proslogion 2 uses this definition for the ontological argument. Gaunilo was his critic, Aquinas rejected the argument, and Augustine is known for the privation theory of evil.

  2. "The order and connection of ideas is the same as the order and connection of things" expresses:

    1. Leibniz's pre-established harmony
    2. Malebranche's occasionalism
    3. Spinoza's parallelism
    4. Descartes's interactionism
    Show answer

    Answer: C — Spinoza's parallelism

    This is Ethics II, proposition 7: mind and body are one thing under two attributes. Pre-established harmony synchronises windowless monads, interactionism has mind and body act on each other, and occasionalism makes God the only true cause.

  3. For Leibniz, truths of fact are governed by the principle of:

    1. Clarity and distinctness
    2. Excluded middle
    3. Non-contradiction
    4. Sufficient reason
    Show answer

    Answer: D — Sufficient reason

    Truths of fact are contingent and rest on sufficient reason; truths of reason are necessary and rest on non-contradiction. Clarity and distinctness is Descartes's criterion of truth.

  4. Descartes's idea of a siren or hippogriff is, in the classification of Meditation III, an idea that is:

    1. Innate
    2. Clear and distinct
    3. Factitious
    4. Adventitious
    Show answer

    Answer: C — Factitious

    Ideas I construct myself are factitious. Adventitious ideas seem to come from outside (the sun as seen), and innate ideas, such as the ideas of God and of mathematical truths, are implanted by God.

  5. Aquinas held that in God, unlike in creatures:

    1. There is potentiality but no actuality
    2. Essence and existence are identical
    3. Essence and existence are really distinct
    4. Existence can be known from the definition alone, so the ontological argument succeeds
    Show answer

    Answer: B — Essence and existence are identical

    In On Being and Essence Aquinas argues that God's essence is his existence (God is subsistent being itself), while in creatures the two are distinct. God is pure actuality, and Aquinas rejects the ontological argument because we do not know God's essence.

  6. Assertion (A): Augustine held that evil is not a substance created by God. Reason (R): For Augustine, evil is a privation of good, as blindness is an absence of sight. Choose the correct option.

    1. Both A and R are true, and R is the correct explanation of A
    2. Both A and R are true, but R is not the correct explanation of A
    3. A is true, but R is false
    4. A is false, but R is true
    Show answer

    Answer: A — Both A and R are true, and R is the correct explanation of A

    Both are true and R explains A. Because evil is a lack rather than a positive being, God, who creates only being and good, is not its author; this was Augustine's answer to the Manichaean view of evil as a rival principle.

  7. Assertion (A): Leibniz denies real causal interaction between mind and body. Reason (R): Leibniz holds that there is only one substance, of which mind and body are two attributes. Choose the correct option.

    1. Both A and R are true, and R is the correct explanation of A
    2. Both A and R are true, but R is not the correct explanation of A
    3. A is true, but R is false
    4. A is false, but R is true
    Show answer

    Answer: C — A is true, but R is false

    A is true: monads are windowless, so mind and body agree by pre-established harmony, not interaction. R is false: one substance with the attributes of thought and extension is Spinoza's view; Leibniz holds there are infinitely many substances, the monads.

  8. Which of the following are among Spinoza's three kinds of knowledge? Select all that apply.

    1. Opinion or imagination
    2. Intuitive knowledge (scientia intuitiva)
    3. Reason
    4. Sensitive knowledge of particular existence
    Show answer

    Answer: A — Opinion or imagination; B — Intuitive knowledge (scientia intuitiva); C — Reason

    Spinoza's three are opinion (imagination), reason and intuitive knowledge. "Sensitive knowledge" is the third of Locke's three degrees of knowledge (intuitive, demonstrative, sensitive), not one of Spinoza's.

  9. Which of the following grounds of doubt does Descartes use in Meditation I? Select all that apply.

    1. The senses sometimes deceive
    2. An evil demon may deceive me
    3. I may be dreaming
    4. Causes are only constant conjunctions
    Show answer

    Answer: A — The senses sometimes deceive; B — An evil demon may deceive me; C — I may be dreaming

    Meditation I doubts through sensory error, the dream argument and the evil demon. The claim that causation is only constant conjunction is Hume's, a century later.

  10. How many "ways" (proofs of God's existence) does Aquinas give in the Summa Theologiae? Type the number.

    Numerical answer — type the value.

    Show answer

    Answer: 5

    Five: from motion, efficient causation, contingency, gradation and governance (design), in Summa Theologiae I, question 2, article 3.