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Enlightenment and knowledge · 1637 onward

Rationalism

Overview

Reason and intellect, rather than sensory experience, are the primary sources of knowledge.

Descartes, Spinoza and Leibniz trust reason to reach knowledge the senses do not earn on their own. The senses are vivid, and they conflict. A tower looks round from far away. Reason, on this account, can find what must be so, and can keep a grip when the look of things shifts.

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Descartes’s method is to doubt until he finds what the doubt itself cannot throw out. A mind that doubts is still there as the one doubting. From that thin certainty he tries to rebuild. Spinoza will not split the world into two substances the way Descartes does. Leibniz will fill it with minds. They share the bet: a chain of reasons can outrank a striking impression.

The risk of the school is a system so pleased with its necessity that it stops checking the case. A rationalist who will not look has become a different kind of dogmatist. The gift, when it stays honest, is the demand that a belief say how it would follow, not only how strongly it is felt.

Where it answers

Key terms

Innate ideas
Ideas the mind has from birth rather than from experience, such as the idea of number.
Clear and distinct ideas
Ideas so evident that, for Descartes, they cannot be doubted.
Substance
What exists independently; Spinoza argued there is only one, God or Nature.
Principle of sufficient reason
Leibniz’s rule that nothing is so without a reason why it is so and not otherwise.

In their words

“I think, therefore I am.”
Descartes, Discourse on the Method, part 4, trans. John Veitch

Further reading

  • Meditations on First Philosophy · René Descartes
  • Ethics (Spinoza)
  • Monadology · Gottfried Wilhelm Leibniz

Learn this

7 lessons on the climb

René DescartesWhat the Doubt Cannot Throw OutThe Cave and the QuestionThe Order of CreationTest Your CertaintyThe Room That Follows RulesThe Brain in a VatUnderstanding Sets You Free

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Quest2Reason. “Rationalism.” https://quest2reason.com/learn/ideas/rationalism. Accessed 13 November 2026.