John Stuart Mill, in his System of Logic (Book III, Chapters 8–10), set forth and discussed five methods of experimental inquiry, calling them the method of agreement, the method of difference, the joint method of agreement and difference, the method of residues, and the method of concomitant variation. Mill maintained that these are the methods by which we both discover and demonstrate causal relationships, and that they are of fundamental importance in scientific investigation. Mill called these methods "eliminative methods of induction." In so doing, he was drawing an analogy with the elimination of terms in an algebraic equation—an analogy that is rather forced, except with respect to the various methods that are classed under the heading of method of difference. As will be demonstrated, it is perhaps best to use the term "eliminative methods" with reference to...