What are some arguments that Immanuel Kant offered in support of his position of free will?
Kant believed that humans have "rational free will," or the ability to make decisions based not on some unseen influence, but on their own deliberate choices. He saw free will as separated from religion, philosophy, and society; people have the ability to choose their actions, and must deal with the consequences as a part of that freedom. Kant also rejected Determinism, the idea that all actions are the result of the events occurring in such a manner that no other action could be taken; Kant believed that humans had the ability to choose any action, good or evil, but that their choice required the ability to accept consequences as they came.
"The will is therefore the faculty of desire considered not so much in relation to action (as choice is) but rather in relation to the ground determining choice in action. The will itself, strictly speaking, has no determining ground; insofar as it can determine choice, it is instead practical reason itself."
(Kant, Metaphysics of Morals, Wikipedia)
In other words, free will is based in personal morality, and that morality itself comes from the ability to judge morals. A person is not acting morally if he has no other choice; if people act from fate or determinism, then moral choices are simply mechanical functions. For an act to be moral, the will (Ego) must be free to see the act and judge its morality. Otherwise, it is nothing more than the product of its environment.
Free will thus resides on a foundation of objective judgement, insofar as any human is objective, and is capable of any action because it must be capable of good and evil to be truly free.