Kant went to all that trouble to preserve the priority of liberty in the idea of justice. Liberty, as experienced in this word, is contingent. The will is inevitably constrained by the nature of material existence. To be the predicate of justice, liberty cannot be contingent. The will must be free of the constraints that material existence inevitably puts on it.

That is what took Kant to transcendentalism. He had to establish a realm in which the will can be utterly autonomous, constrained by nothing but rules of governance it would determine for itself — which leads to his famous “categorical imperative.”