To approach the many challenges involved in the notion of engineering concepts, it is important to have a clear idea of the starting point—the concepts that people use in their everyday lives, in conversations and in expressing beliefs, desires, intentions and so forth. The first Section of this chapter introduces evidence that I have accumulated over the last many years concerning the flexibility, context-dependence, and vagueness of such common concepts. The concept engineer needs to understand the structure of the “raw material” with which she is working.