  The first goal is to emphasise how each theory leaves us in a position in which the succeeding one appears as a perfectly natural next step in the development of ideas (McCrea 1939, p. 137). McCrea embeds this argument in an account of how analogue models (à la Campbell) are used to set up new theories—this is essentially an account of how discovery might proceed in linking an older theory with its successor. The second goal is to show how, in spite of superficial differences in character, the theories in question all necessarily possess the same general structure constituted by the presence of hypotheses, from which certain general mathematical relations are deduced, which in their turn are used to predict relations between observable quantities. As McCrea notes, “this study may claim an interest of its own, but it is presented also for a further reason” namely, that