 Once specifications of all relevant quantities are made, empirical claims are possible. There are, in Reichenbach’s view, two related ambiguities. First, different stipulations about measurement can lead to apparently different empirical generalizations that nevertheless are empirically indistinguishable—although just what “empirically indistinguishable” can consistently mean for Reichenbach is problematic for reasons to be noted. Second, the same total theory may be partitioned in more than one way into claims true by stipulation and empirical claims. Reichenbach’s solution to these forms of underdetermination is that “equivalent theories” say the same thing. The equivalence relation he intends is unclear. In earlier writings he suggests that empirically equivalent theories are those that have the same empirically testable consequences; a later formulation is that empirically equivalent theories are those that have the same (posterior) probability on any observations. The later characterization is as clear as Reichenbach’s characterization of the probability of theories and their confirmation (see below). The earlier characterization results in a semantics for which no effective proof theory is possible (Glymour, 1970). 