There are not many professions where people routinely ask you to justify your work, but theoretical physics is one of them. In the wake of the recent Nobel Prize to Peter Higgs and François Englert for their research on the Higgs Boson, this scrutiny has intensified. Richard Feynman once quipped, “Physics is like sex. Sure, it may give some practical results, but that’s not why we do it.” I will attempt a more comprehensive answer . Theoretical physicists construct theories of nature. For a theory to be true it must be both consistent with itself and consistent with nature. The first aspect can be verified with mathematics, the second with experiment. Thus physics is based on a checks-and-balance system between these two approaches. Without theorists, experimentalists would not have anything to test. Without experimentalists, theorists would not have anything to explain. I personally chose theory because experimental physics appeared too difficult. This process of scienti...