Rationalism
rationalist · a priori knowledge
The thesis that reason, not experience, is the primary source of substantive knowledge.
In practice
Before running a test, you can often rule out an option by reasoning that it contradicts something already established. That step is a priori, and it saves the cost of an experiment that could only confirm the contradiction.
The common mistake
Treating rationalism as a preference for thinking over evidence. The claim is narrower: that certain truths are established by reasoning, and that experience alone cannot account for the structure the mind brings to it.
Rationalism holds that reason can reach substantive truths about reality without depending on sense experience. Some knowledge is a priori: available in advance of observation, and more secure than observation could make it.
The continental line
Descartes supplies the model. In the Meditations (1641) he doubts everything that can be doubted, including the senses, and finds one thing that survives — that he is thinking, and therefore exists. The cogito is reached by reasoning alone, and he builds outward from it rather than from observation.
Spinoza wrote the Ethics (1677) in the form of geometry, deriving claims about God, nature and the emotions from definitions and axioms. Leibniz argued that everything that is the case has an explanation, the principle of sufficient reason, and treated that principle as known by reason.
Innate ideas
Rationalists generally hold that the mind brings something to experience. Descartes treated the idea of perfection as innate, since no imperfect thing encountered in experience could have produced it. Locke attacked the doctrine directly, which is the point where the dispute with empiricism becomes explicit.
What survives
Few philosophers now defend the strong claim that reason alone discloses the structure of the world. The weaker claim has fared better: that some knowledge is a priori, that logic and mathematics are not generalizations from observation, and that the mind is not a blank sheet. Work in linguistics on the structure children bring to language has given the last of these an empirical form its early defenders would not have expected.
Concept web
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What is rationalism in philosophy?
The view that reason is a source of substantive knowledge about reality, independent of sense experience. Rationalists typically hold that some ideas are innate and that a priori reasoning can establish truths about the world.
Who were the main rationalist philosophers?
Descartes, Spinoza and Leibniz are the classical figures, often grouped as the continental rationalists in contrast with the British empiricists Locke, Berkeley and Hume.
What is an a priori truth?
A truth knowable independently of particular experience, such as a mathematical theorem. A posteriori truths, by contrast, require observation to establish.