 “[W]e may discover, by mere observation without experiment, a real uniformity in nature” (System, VII: 386). Where possible, however, engaging in experimental activity is the most direct way to uncover the causal relations between events, because it allows us to “meet with some of the antecedents apart from the rest, and observe what follows from them; or some of the consequents, and observe by what they are preceded” (System, VII: 381). Mill claims that, as science has progressed, four methods have emerged as successful in isolating causes of observed phenomena (System, VII: 388–406; see Cobb 2017: 240–1). Firstly, the Method of Agreement: where instances of phenomenon A are always accompanied with phenomenon a, even when other circumstances accompanying A are varied, we have reason to believe that A and a are causally related. Secondly, the Method of Difference: where the only distinguishing feature marking situations in which phenomenon a occurs or does not occur is the presence or absence of phenomenon A, there is reason to think that A is an indispensable part of the cause of a. (If we have noted, via the Method of Agreement, that in all instances of A, a is present, we can, where possible, systematically withdraw A, to determine whether A is a cause of a by the Method of Difference. Mill terms this the Joint Method of Agreement and Difference.) Thirdly, the Method of Residues: against the knowledge that A is the cause of a, and B the cause of b, where ABC causes abc, and AB causes merely ab, we can (by ruling out that c is the joint effect of AB) regard C as the cause of c. Fourthly, the Method of Concomitant Variations: whenever a varies when A varies in some particular manner, a may be thought to be causally connected to A.