Rationalism

Rationalism claims that reason is a source of substantive knowledge in its own right.^[1] Not that the senses are worthless — that no one holds — but that some truths are established a priori, without observation, and are none the worse for it. Mathematics is the standing exhibit: nobody checks whether the angles of a triangle sum to two right angles by measuring triangles, and a counterexample wouldn't be believed if reported.

The strong versions go further: that we have innate ideas the mind brings to experience rather than takes from it, and that reason can reach conclusions about what exists. Descartes' method is the emblem — doubt everything doubtable, find the one thing that survives (I think, therefore I am), and rebuild from there by deduction. The weakness is visible in the same example: the rebuilding needs a premise the doubt was supposed to have suspended, and Descartes' proofs of God have convinced few since.

The modern settlement, if there is one, is Kant's: experience supplies the content, the mind supplies the form, and neither alone yields knowledge. That is also why transcendental idealism sits where it does — it is an epistemological answer with metaphysical consequences.