Experimental Reason and Freedom

A main concern of Kant's is to establish that the Principle of Pure Practical Reason is a sufficient condition of Freedom.  In contrast, he seems less conscientious about whether or not that Principle is a necessary condition of Freedom, as well.  Thus, he seems to present no a priori argument against the possibility that Experimental Reason is another ground of Freedom, i. e. that the question 'What if?' can spontaneously initiate an efficacious course of action.