Which kinds of knowledge do rationalists describe as a priori?

In the context of rationalism, a priori knowledge refers to truths that are acquired independently of sense experience, relying instead on the intellect's ability to grasp logical structures directly. Rationalists maintain that such knowledge possesses both certainty and universality, characteristics they argue cannot be derived from empirical observation.123

Rationalists identify a priori knowledge across several fundamental domains:

• Logic and Mathematics: These fields provide the most prominent examples of a priori truths, such as the principle that two straight lines cannot enclose a space or that $2+2=4$.32
• Metaphysics and Ethics: Rationalists assert that certain principles in these areas are so fundamental that denying them leads to logical contradiction.2
• Innate Concepts: Many rationalists, such as René Descartes, hold that the concepts required for a priori knowledge are innate—part of the original structure of the human mind.14

These truths are often apprehended through intellectual intuition, the direct grasp of self-evident propositions, or through deductive reasoning.4