 Marburg philosophy of science thus includes the claim that there could be rival scientific theories, each of which is empirically  adequate.[14]  Given this, in what sense is science objective? How is there unity in science, when there are no theory-neutral experiments that would prevent different scientists from adopting different theories? Cassirer (1910) in particular proposed an intricate answer to this question (Heis 2014). He argued that there is a common set of demands on theories that will constrain the choice of theory. These demands are a priori, in a regulative sense, and partially constitute what he calls the “form” of a scientific theory. These demands include that a theory be simple and of wide  scope.[15]  He also argues that even in the face of radical conceptual changes and shifts in ontology, some features of scientific theories are maintained—for instance the approximate mathematical structure of the theory.