A theory of conceptual development must specify the innate representational primitives, must characterize the ways in which the initial state differs from the adult state, and must characterize the processes through which one is transformed into the other. I will defend three theses. With respect to the initial state, the innate stock of primitives is not limited to sensory, perceptual, or sensory-motor representations; rather, there are also innate conceptual representations. With respect to developmental change, conceptual development consists of episodes of qualitative change, resulting in systems of representation that are more powerful than, and sometimes incommensurable with, those from which they were built. With respect to a learning mechanism that achieves conceptual discontinuity, I offer Quinian bootstrapping. I will illustrate these theses with a case study of the origin of concepts of natural number.