Classically Kant’s Critical philosophy goes beyond the duality of empiricism and rationalism of the philosophies to which he was referring that was the context of his work. He believed that Reason produced phantasms on its own without the input of experience. So he believed there was limits to the usefulness of Reason, but still thought that if Reason was used properly, eg. for science, then there was great benefit in it. He was looking for an alternative to either induction (empiricism) and deduction (rationality). It was not till much later that Peirce produced abduction as the third alternative that allowed one to formulate hypothesis in science that would allow the connection between theory and experimentation through the projection of hypotheses. Without that idea, Kant basically held that we were presented with singular a priori syntheses that bridged the gap between a priori deduction and a posteriori induction. These a prior syntheses were the projection of spacetime and the categories. It was these projections that allowed us to locate things in spacetime in our experience and to identify objects causally related in experience and to make out of that an objective story. Basically it was recognizing the limits of the possibility of experience that allowed one to assume a stance toward transcendental realism. His position was that only though transcendental idealism could view of transcendental realism be achieved. Basically we have been operating in science within the boundaries that Kant set ever since.
Kant was a rationalist that recognized the limits of what reason could accomplish on its own. And specified that only reason operating on the basis of experience could be trusted to tell us something important about the world in which we live, i.e. something scientific.