Rationalism holds that reason is the primary source of knowledge. It does not deny the senses, but it argues that reason can grasp truths that experience cannot. Mathematics is the model. You do not need to measure triangles to know that their angles sum to 180 degrees. You can prove it. Rationalists believe that some truths are innate or discoverable by pure thought. Descartes, Spinoza, and Leibniz are the classic rationalists. They argued that the senses are unreliable and that only reason can provide certain knowledge.

Rationalism contrasts with empiricism, which holds that knowledge comes from experience. Kant tried to synthesize the two, arguing that knowledge requires both reason and experience. Reason provides the categories that structure experience. Experience provides the content. Rationalism has been influential in mathematics, logic, and metaphysics. It has also been criticized. If reason is the source of knowledge, how do we know which reasoning is correct? Rationalists reply that reason is self-correcting. It can identify and fix its own errors. The debate between rationalism and empiricism is one of the oldest in philosophy, and it continues in contemporary epistemology and philosophy of mind.

Rationalist themes

• Innate ideas. Some knowledge is inborn.
• Deduction. Reason can derive truths from first principles.
• Certainty. Reason can provide indubitable knowledge.
• Mathematics as model. The method of geometry applies to philosophy.

Rationalism is not the same as rationality. Rationality is the capacity to reason. Rationalism is a philosophical position about the source of knowledge.