  Many issues in the philosophy of science concern the nature of theories and certain relations that may obtain between them. Typically, one is interested in the degree to which a successor to a given theory “goes beyond” (both descriptively and explanatorily) the theory it succeeds. Most often these issues are framed in the context of reductive relations between theories. When does a theory \(T'\) reduce to a theory \(T\)? How is one to understand the nature of this reduction relation? Interestingly, there are two distinct, yet, related ways of understanding the reductive relationship between \(T\) and \(T'\). Thomas Nickles noted this in a paper entitled “Two Concepts of Intertheoretic Reduction.” On the one hand, there is the “philosopher’s” sense of reduction on which the supplanted theory is said to reduce to the newer more encompassing theory. On the other hand, the “physicist’s” sense of reduction puts things the other way. The newer, typically more refined theory is said to reduce to the older typically less encompassing theory in some sort of limit. These two senses of reduction will be discussed in turn. 