Rationalists believed you could discover truths without getting off the couch
Can you know something for certain without looking at the world? Rationalists say yes. The number three is prime and bigger than two, so a prime greater than two exists, and no experiment was needed to learn it. The philosopher Galen Strawson summed up the idea as knowledge gained while lying on your couch.
Rationalism holds that reason, rather than faith, tradition or the senses, is the chief source and test of knowledge. Positions range from the moderate claim that reason takes precedence over other routes to the extreme view that it is the only path. During the Enlightenment it faced off against empiricism: René Descartes stressed that much knowledge is innate and the intellect can grasp logical truths directly, while John Locke argued that knowledge comes mainly from careful observation of the world.
Descartes, Baruch Spinoza and Gottfried Leibniz brought mathematical method into philosophy, a movement later labelled continental rationalism because it dominated mainland Europe while Britain leaned empiricist. Spinoza and Leibniz thought that, in principle, all knowledge, scientific knowledge included, could be deduced from basic axioms, as geometry is. Both admitted that humans could manage this in practice only in areas like mathematics, and Leibniz conceded that in three-quarters of our actions we behave as mere empiricists. The sharp rationalist versus empiricist split was drawn by later historians; Descartes and Locke actually held similar views about ideas.
To count as a rationalist, one must accept at least one of three claims: that some truths are known by intuition and deduction, that some knowledge is innate, or that some concepts are innate. Leibniz's argument for the first was that however many examples confirm a general rule, they never prove it must always hold; the necessary truths of arithmetic and geometry therefore need a foundation beyond the senses. Rationalists usually reject scepticism about what they say can be known in advance, and many adopt foundationalism, the idea that some beliefs need no support from others.
The word has also had a social life. In the seventeenth and eighteenth centuries rationalist was often a slur for anti-religious free thinkers; in 1670 one writer sneered that a mere rationalist was an atheist by another name. The philosopher John Cottingham notes that terms like humanist or materialist have largely replaced that usage.
Source: Rationalism
