The remarkable features of quantum theory are best appreciated by comparing the theory to other possible theories-what Spekkens calls "foil" theories [1]. The most celebrated example of this approach was Bell's analysis [2], which showed that entangled quantum systems have statistical properties unlike any hypothetical local hidden variable model. More recently, there have been several efforts to give quantum theory an operational axiomatic foundation [3,4,5]. In these efforts, a general abstract framework is posited to describe system preparations, choices of measurement, observed results of measurement, and probabilities. Many possible theories can be expressed in the framework. The axioms embody fundamental aspects of quantum theory that uniquely identify it among them. A striking lesson of this work is that familiar quantum theory can be characterized by axioms that seem to have little to do with the traditional quantum machinery of states and observables in Hilbert space. The Hilbert space structure is "derived" from the operational axioms.These approaches are based on two distinct concepts of generalization. First, we generalize within quantum theory to give the theory its most general form. For example, we generalize state vectors to density operators as a