As I am tired of the scattering of side-problems of kantian philosophy that have to be seen in the light of the very idea behind kantian philosophy, I want to create a question covering the interpretational frameworks that are present here.
As Eckart Förster states in the prologue of his The 25 Years of Philosophy, Kant himself formulates a first sketch of critical philosophy in his Herz-letter (1772), from which I will translate a central statement by myself:
I asked myself: What is the foundation of/reason for [Grund] the relation of the perception [Vorstellung] to its object [Gegenstand]? (10:130)
Förster continues that the foundation of/reason for the relation for two situations seems to be unproblematic: Sensing means that the object invokes the perception, moral acting means invoking an object from a perception. But how can we possibly imagine a relation between a metaphysical object and a perception so that we can judge the truth-value (that is understanding the relation)? How is metaphysics possible? Another translation of Kant's own thoughts on that:
Where do these things [Dinge] come from if not through the way they affect [affizieren] us, and if these intellectual perceptions [intellektuelle Vorstellungen] rest on our internal activities, how come the correspondance they ought to have with the objects they are not invoked by? (10:131)
The problem we are confronted with, as philosophers, is grave: As nature is best described by natural sciences, how can we claim that we know anything not accessible by them?
His answer obviously is transcendental philosophy and transcendental idealism.
To end the question with the questions:
1) What philosophical schools of thought (then and now) are Kant's enemies? What are his objections against them?
2) What are his main arguments for transcendental idealism?
3) Why is he so desperate in defending freedom?
Each of them may be answered for itself, or, even better, in the light of each other. I beg you to found your answers on sources. I will answer myself in about two weeks after ending my paper about the practical part of the question.