German idealism
Neo-Kantianism refers broadly to a revived type of philosophy along the lines of that laid down by Immanuel Kant in the 18th century, or more specifically by Schopenhauer's criticism of the Kantian philosophy in his work "The World as Will and Representation" (1818), as well as by other post-Kantian philosophers such as Jakob Friedrich Fries and Johann Friedrich Herbart. It has some more specific reference in later German philosophy.
Immanuel Kant
Kant believed that mathematical truths were forms of synthetic a priori knowledge, which means they are necessary and universal, yet known through intuition. Kant's often brief remarks about mathematics influenced the mathematical school known as intuitionism, a movement in philosophy of mathematics opposed to Hilbert's formalism, and Frege and Bertrand Russell's logicism.