Rationalism

Philosophy · Modern philosophy. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Rationalism holds that reason, not the senses, is the chief source of certain knowledge. Descartes doubted everything that could be doubted: the senses, the waking world, and even mathematics, by supposing an evil demon. One truth survived, the cogito: I think, therefore I am. From there he argued to God, and from God's truthfulness to the reliability of clear and distinct ideas.

Descartes held that mind (thinking) and body (extended) are two different substances, which raises the problem of how they interact. Spinoza answered that there is only one substance, God or Nature, with thought and extension as its attributes. Leibniz answered that reality consists of countless non-interacting monads, coordinated by God.

Rules to remember

Descartes (1596 to 1650): Discourse on Method; Meditations on First Philosophy (1641)
Criterion of truth: clear and distinct perception, guaranteed by a God who does not deceive
Descartes's proofs of God: the trademark argument (the idea of a perfect being must come from a perfect being) and an ontological argument (existence belongs to God's perfection)
Dualism: mind and body interact, Descartes suggested, through the pineal gland
Spinoza: Ethics, set out in geometrical order; monism; mind and body are one and the same thing, conceived under two attributes (thought and extension) of the one substance
Leibniz: Monadology; monads are simple, non-material and 'windowless'
Pre-established harmony: God set all monads at creation so that their states agree without acting on one another

Common mistake

Treating the method of doubt as scepticism. Descartes doubts in order to find a certain foundation; doubt is his tool, not his conclusion.

Worked example

Three philosophers explain why my arm rises when I decide to raise it. X: the mind acts on the body. Y: decision and movement are one event under two attributes. Z: mind and body run in parallel like two clocks set to agree. Name X, Y and Z.

X accepts two substances that really interact: this is Descartes's interactionist dualism.
Y denies two substances and speaks of one reality under two attributes: this is Spinoza's monism.
Z denies interaction and appeals to an agreement arranged in advance: this is Leibniz's pre-established harmony.

Answer: X = Descartes, Y = Spinoza, Z = Leibniz.

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