 Mill maintains that direct inductive methods cannot be used to study phenomena in which many causal factors are in play. If, for example, one attempts to investigate whether tariffs enhance or impede prosperity by comparing the prosperity of nations with high tariffs and nations without high tariffs, the results will be uninformative, because prosperity depends on so many other causal factors. So, Mill argues, one needs instead to employ the method a priori. Despite its name, this too is an inductive method. However, unlike the method a posteriori, the method a priori is an indirect inductive method. Scientists first determine the laws governing individual causal factors in domains in which Mill’s methods of induction are applicable. Having then determined the laws of the individual causes, they investigate their combined consequences deductively. Finally, there is a role for “verification” of the combined consequences, but owing to the causal complications, this testing has comparatively little weight. The testing of the conclusions serves only as a check on the scientist’s deductions and as an indicator of whether there are significant disturbing causes that scientists have not yet accounted for.