Every branch of science I’ve examined has an equilibrium principle. This is the principle that says, essentially, if nothing changes, then nothing will change. It sounds trite, but it’s essentially the claim that when things stay the same, they don’t require explanation. It’s only when things change that we need to start paying attention. Inertia is an equilibrium principle, and applied forces explain changes in inertia. Inertia itself doesn't require an explanation--it's axiomatic. This is what I mean when I say that science is the study of deviant behavior—it’s all about how things change.