While there are multiple sources of constraint that children can exploit to determine the meaning of a novel word, the biggest source of constraint comes from our conceptual systems. Our conceptual systems predispose us to think of things in some ways, but not in others. Thus, while it is natural to think of a given entity as a dog, animal, living thing, or pet, it is not natural to think of the same entity as a furry brown thing, brown four-legged thing, or as undetatched dog parts. This is presumably why the former, but not the latter, are likely meanings for novel words. In this talk, I present a theory of a fragment of our conceptual system that is particularly important for early lexical development AD our conception of entities as objects and stuff -- and show how it can be used to explain a number of findings in lexical development. In addition, I will show how the proposed theory can account for a range of conceptual phenomena including the naturalness of certain concepts. Finally, I say a few words about how this approach to the study of conceptual structure, which regards our conceptual system as being constituted by a formal system of knowledge, is related to other approaches, and how it can be developed to account for a greater range of concepts.