 Cohen argues that there are three different “levels” or “degrees” of Kant’s a priori. He claims that the first of these is inessential for Kant’s philosophy, and consists in the apparently permanent “metaphysical” structures we can discover in our own thought by means of introspection. The second level of the a priori consists in the forms of sensibility and the understanding, that is, space, time, and the categories. But the necessity of these forms ultimately derives from the necessity of the third level of the a priori, because the forms of sensibility and understanding are “scientific abstractions” (Cohen 1987 [1871b], 83–84) from that third level. The third level is thus the most important of the three levels. Cohen argues that a priori laws in this most important, third sense consist in the “formal conditions of the possibility of experience.” Or, as he also sometimes puts it, the a priori in this third sense consists in laws that are “constitutive of” of the possibility of experience. For Cohen, these necessary, a priori laws define what an object of experience is for us. (Klaus Christian Köhnke identifies this point in Cohen as the source of the central Marburg School doctrine that a priori laws “generate” objects of possible experience [Köhnke 1991, 178–84].)