The rationalist intuition comes from noticing something odd about mathematics. You know that the interior angles of a triangle sum to two right angles, and you did not establish it by measuring triangles — measurement could never establish that it holds necessarily, for every triangle, including ones nobody has drawn. Something other than observation is doing the work. Rationalism is the project of finding out what, and how far it extends.
Descartes sought a foundation immune to doubt. The senses sometimes deceive; dreams are indistinguishable from waking; a powerful deceiver could even corrupt reasoning. What survives is that something is doing the doubting — the cogito. From this he attempted to rebuild, arguing that clear and distinct perceptions can be trusted. The reconstruction is widely regarded as less successful than the demolition, particularly because it leans on an argument for God's existence to guarantee that our faculties are not systematically misleading, which many read as circular.
Spinoza took the mathematical model most literally, writing his Ethics in geometrical form — definitions, axioms, propositions, demonstrations — on the view that reality has a rational structure reason can trace. His conclusions were radical: there is only one substance, which may be called God or Nature, and everything else is a modification of it. Whatever one makes of the system, it shows what rationalism looks like when pursued without compromise.
Leibniz defended innate ideas against Locke with an image worth keeping. The mind is not a blank tablet but like a block of marble whose veins already favour one figure over another — not containing the finished statue, but disposed toward it. This is a more careful claim than the one Locke attacked: innate ideas need not be consciously present from birth, only latent and drawn out by experience. He also distinguished truths of reason, which are necessary and whose denial is contradictory, from truths of fact, which are contingent — a distinction that has proved durable.
ObjectionsWhere it strains.
Systems without checks. The characteristic rationalist risk is constructing something internally consistent and disconnected from how the world actually is. Rationalist metaphysics produced several elaborate systems that disagree with each other, and reason alone gave no way to adjudicate — which is exactly what empiricists predicted.
Where do innate ideas come from? Positing built-in content raises the question of what makes it reliable. If evolution installed our concepts, they were selected for usefulness rather than truth, which weakens the claim that reason gives privileged access to reality.
Kant's reframing. The most influential response accepts something from each side. Kant argued that all knowledge begins with experience but does not all arise from it: the mind supplies the structure — space, time, causation — while experience supplies content. This preserves necessary knowledge, which was the rationalist's demand, at the cost of restricting it to appearances rather than reality in itself.
The labels have since fallen out of use as team names, but the underlying question is very much alive — it survives in debates about innate cognitive structure, whether any knowledge is genuinely a priori, and how much of what we take ourselves to observe is contributed by the observer.