Hence the wave theory should be inferred as the best explanation.The above arguments exemplify three important criteria for determining the best explanation. By "criteria" I do not mean necessary or sufficient conditions. We shall see that the complexity of scientific reasoning precludes the presentation of such conditions of the best explanation. A criterion is rather a standard of judgment which must be weighed against other criteria used in evaluating explanatory hypotheses. The tensions between the three main criteria will be described below. I call the three criteria consilience,simplicity, and analogy.The notion of consilience is derived from the writings of William Whewell Consilience is intended to serve as a measure of how much a theory explains, so that we can use it to tell when one theory explains more of the evidence than another theory. Roughly, a theory is said to be consilient if it explains at least two classes of facts. Then one theory is more consilient than another if it explains more classes of facts than the other does. Intuitively, we show one theory to be more consilient than another by pointing to a class or classes of facts which it explains but which the other theory does not.To get a more precise definition, let T be a theory consisting of a set of hypotheses .H1.. . Hn}; let A be a set of auxiliary hypotheses JA1... A.A}; let C be a set of accepted conditions IC,. . Cj}; and let F be a set of classes of facts {F1. . .F7,J. Then T is consilient if and only if T, in union with A and C, explains the elements of the Fi, for k > 2.To get the comparative notion, let FTf be the set of classes of facts explained by theory Ti. Then we can choose between two different definitions of comparative consilience: (1) T1 is more consilient than T2 if and only if the cardinality of FT1 is greater than the cardinality of FT2; or (2) T1 is more consilient than T2 if and only if FT2 is a proper subset of FT1. These definitions are not equivalent, because FT1 might be much larger than FT2, while atthe same time there are a few elements of FT2 that are not in FT1.In other words, it is possible that T1 explains many more classes of facts than T2, but that there are still some facts that only T2 explains." In cases where these two definitions do not coincide, decisions concerning the best explanation must be made according to what theory explains the most important facts, or on the basis of other criteria discussed below.