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.
Theodor Birt
When Birt was a professor in classics at Marburg University, its philosophy department was among the most important in Europe. Hermann Cohen and Paul Natorp there developed what came to be known as Neo-Kantianism. As the name suggests, this aimed to develop Kant's philosophy and apply it to a wide range of contemporary issues. In addition to Cohen and Natorp, Marburg's philosophy department was home during Birt's career to such important figures as Martin Heidegger, Hannah Arendt, Paul Friedlander, Ernst Cassirer, and José Ortega y Gasset. Birt contributed a section to one of Natorp's books, who thanked him as a 'great friend and colleague.' A letter from Kurt Wolff, who became a well-known German publisher, to the writer Boris Pasternak shows that Birt and the philosophers were part of the same social circle: