The previous section considered the geometrical aspects of wave propagation and how they relate to seismic imaging. Here we consider all the surrounding disciplines, how they relate, how they fail to relate to our central topic of earth imaging. Physics, the kind of physics one learns as an undergraduate, has a relevance to earth imaging, and also an irrelevance--the simple models don't always work. Likewise, mathematics, with its precise problem formulations is both relevant, and irrelevant to earth imaging. And geophysics itself, with its classic areas of theory and observation is both relevant and irrelevant to practical earth imaging. Finally geology brings insights as to why and why not basic physics is a realistic guide to practice. In all the surrounding disciplines of physics, mathematics, geophysics, and geology, I will give you some indication of the degree to which theory can be used as a guide by pointing out some inconsistencies between theory and current industrial practice.