 Whewell discussed a further, related test of a theory’s truth: namely, “coherence.” In the case of true theories, Whewell claimed, “the system becomes more coherent as it is further extended. The elements which we require for explaining a new class of facts are already contained in our system….In false theories, the contrary is the case” (1858b, 91). Coherence occurs when we are able to extend our hypothesis to colligate a new class of phenomena without ad hoc modification of the hypothesis. When Newton extended his theory regarding an inverse-square attractive force, which colligated facts of planetary motion and lunar motion, to the class of “tidal activity,” he did not need to add any new suppositions to the theory in order to colligate correctly the facts about particular tides. On the other hand, Whewell explained, when phlogiston theory, which colligated facts about the class of phenomena “chemical combination,” was extended to colligate the class of phenomena “weight of bodies,” it was unable to do so without an ad hoc and implausible modification (namely, the assumption that phlogiston has “negative weight”) (see 1858b, 92–3). Thus coherence can be seen as a type of consilience that happens over time; indeed, Whewell remarked that these two criteria—consilience and coherence—“are, in fact, hardly different” (1858b, 95).