Causality

In his monumental A System of Logic Ratiocinative and Inductive (1843), John Stuart Mill (1806–1873) defended the Regularity View of Causality, with the sophisticated addition that in claiming that an effect invariably follows from the cause, the cause should be taken to be the whole conjunction of the conditions that are sufficient and necessary for the effect. For Mill, regular association is not, on its own, enough for causality. A regular association of events is causal only if it is "unconditional"—that is, only if its occurrence does not depend on the presence of further factors which are such that, given their presence, the effect would occur even if its putative cause was not present. A clear case in which unconditionality fails is when the events that are invariably conjoined are effects of a common cause. Ultimately, Mill took to be causal those invariable successions that constitute laws of nature.

Mill is also famous for his methods by which causes can be discovered. These are known as the Method of Agreement and the Method of Difference. According to the first, the cause is the common factor in a number of otherwise different cases in which the effect occurs. According to the second, the cause is the factor that is different in two cases, which are similar except that in the one the effect occurs, while in the other it does not. In effect, Mill's methods encapsulate what is going on in controlled experiments: we find causes by creating circumstances in which the presence (or the absence) of a factor makes the only difference to the production (or the absence) of an effect. Mill, however, was adamant that his methods work only if certain metaphysical assumptions are in place. It must be the case that: (a) events have causes; (b) events have a limited number of possible causes; and (c) same causes have same effects, and conversely.