Rationalism was a cornerstone of early-modern European philosophy, flourishing in the 17th and 18th centuries. Its thinkers argued that reason, rather than sensory experience, is the surest path to knowledge — that some truths are grasped by the mind alone. René Descartes captured its spirit with “I think, therefore I am,” while Spinoza built an entire ethics on geometric logic and Leibniz pursued a universal calculus of reason. It defined itself against Empiricism, which trusted the evidence of the senses.