Comment: Prepared as a chapter for the HANDBOOK OF COMPLEXITY ECONOMICS being edited by P. Chen, W. Elsner, and A. PykaWhat would you do if you were asked to "add" knowledge? Would you say that "one plus one knowledge" is two "knowledges"? Less than that? More? Or something in between? Adding knowledge sounds strange, but it brings to the forefront questions that are as fundamental as they are eclectic. These are questions about the nature of knowledge and about the use of mathematics to model reality. In this chapter, I explore the mathematics of adding knowledge starting from what I believe is an overlooked but key observation: the idea that knowledge is non-fungible.