 By the end of the nineteenth century, it was obvious to any informed observer of science that its categorial structures were in fact hypothetical and dynamic: the fact of scientific experience could no longer be taken as the essentially complete edifice of Newtonian physics, as Kant had done. In Natorp’s rewording of Kant, science is not a factum at all, but a fieri, i.e., not an accomplished deed, but an ongoing doing (cf. Natorp 1910c: 14). Hence, what makes science scientific—i.e., productive of genuine knowledge—cannot possibly be founded on a set of fixed (physical) principles, analogous to mathematical axioms, the certainty of which permanently grounds its theorems. Instead, the Marburgers argue, the essence of science can only reside in its method, i.e., in the regular and regulated manner of its progress. And since this scientificity is equivalent to objectivity or lawfulness, transcendental critique must determine the relation of lawfulness to method.