 EPR now proceed to establish the two premises, beginning with a discussion of the idea of a complete theory. Here they offer only a necessary condition; namely, that for a complete theory “every element of the physical reality must have a counterpart in the physical theory.” The term “element“ may remind one of Mach, for whom this was a central, technical term connected to sensations. (See the entry on  Ernst Mach.)  The use in EPR of elements of reality is also technical but different. Although they do not define an “element of physical reality” explicitly (and, one might note, the language of elements is not part of Einstein’s usage elsewhere), that expression is used when referring to the values of physical quantities (positions, momenta, and so on) that are determined by an underlying “real physical state”. The picture is that quantum systems have real states that assign values to certain quantities. Sometimes EPR describe this by saying the quantities in question have “definite values”, sometimes “there exists an element of physical reality corresponding to the quantity”. Suppose we adapt the simpler terminology and call a quantity on a system definite if that quantity has a definite value; i.e., if the real state of the system assigns a value (an “element of reality”) to the quantity. The relation that associates real states with assignments of values to quantities is functional so that without a change in the real state there is no change among values assigned to quantities. In order to get at the issue of completeness, a primary question for EPR is to determine when a quantity has a definite value. For that purpose they offer a minimal sufficient condition (p. 777):