 The formal route was inaugurated by Popper (1972: 231–236), who defined relative orderings of “verisimilitude” (literally, “likeness to truth”) between theories in a given domain over time by means of a comparison of their true and false consequences. D. Miller (1974) and Tichý (1974) proved that there is a technical problem with this account, however, yielding the consequence that in order for theory A to have greater verisimilitude than theory B, A must be true simpliciter, which leaves the realist desideratum of explaining how strictly false theories can differ with respect to approximate truth unsatisfied (see also Oddie 1986a). Another formal account is the possible worlds approach (also called the “similarity” approach), according to which the truth conditions of a theory are identified with the set of possible worlds in which it is true, and “truth-likeness” is calculated by means of a function that measures the average or some other mathematical “distance” between the actual world and the worlds in that set, thereby facilitating orderings of theories with respect to truth-likeness (Tichý 1976, 1978; Oddie 1986b; Niiniluoto 1987, 1998; for critiques, see D. Miller 1976 and Aronson 1990). One last attempt to formalize approximate truth is the type hierarchies approach, which analyzes truth-likeness in terms of similarity relationships between nodes in tree-structured graphs of types and subtypes representing scientific concepts on the one hand, and the entities in the world they putatively represent on the other (Aronson 1990; Aronson, Harré, &amp; Way 1994: 15–49; for a critique, see Psillos 1999: 270–273).