Propositions are no more constitutive of science than they are of any activity: a body of knowledge is not all there is to the life of science. Thus I take the premise underlying the topic of this symposium to be uncontroversial, there is a “non-propositional” side of science and of biology in particular. From time to time, however, philosophers ask whether the “non-propositional” side of science is theoretically superfluous, or as Duhem put it, logically dispensable. What they mean to ask is whether science can be fully analyzed in propositional terms; must philosophy of science, in other words, consider the non-propositional side in order to adequately account for science? In this paper I explore the boundary between the propositional and non-propositional sides of biological science, drawing on three cases with which I am familiar. It is important to stress that I focus only on the boundary between two perspectives on scientific theory, as exemplified by biological theories. I do not aim to address the larger questions of the non-propositional character of scientific practice tout court. I propose a picture of material model-building in biology in which manipulated systems of material objects function as theoretical models. Sometimes material models serve as direct theoretical models, as in non-mathematized fields of biology where structure may be abstracted directly from the material system without detour through a formal sentential apparatus. Sometimes material models serve as vicarious, or indirect, theoretical models. That is, work with material systems can serve as an important basis for manipulation in thought of abstract, formal objects for the purpose of articulating a theory.