In this article, I give a counterexample to a claim made in (Norton [2008]) that empirically equivalent theories can often be regarded as theoretically equivalent by treating one as having surplus structure, thereby overcoming the problem of underdetermination of theory choice. The case I present is that of Lorentz’s ether theory and Einstein’s theory of special relativity. I argue that Norton’s suggestion that surplus structure is present in Lorentz’s theory in the form of the ether state of rest is based on a misunderstanding of the role that the ether plays in Lorentz’s theory, and that in general, consideration of the conceptual framework in which a theory is embedded is vital to understanding the relationship between different theories.

3.  Background to Lorentz’s and Einstein’s Theory

4.  The Ether State of Rest Is Not Surplus

5.  Does Lorentz’s Theory Have Surplus Structure?

6.  Consequences

6.1.  Earman’s principle

6.2.  What makes Einstein’s theory superior to Lorentz’s theory?