Neo-Kantianism
In late modern Continental philosophy, Neo-Kantianism () was a revival of the 18th-century philosophy of Immanuel Kant. More specifically, it was influenced by Arthur Schopenhauer's critique 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.
Critique of Pure Reason
Kant decided to find an answer and spent at least twelve years thinking about the subject. Although the "Critique of Pure Reason" was set down in written form in just four to five months, while Kant was also lecturing and teaching, the work is a summation of the development of Kant's philosophy throughout that twelve-year period.