method of agreement
the first of the five canons of empirical science laid down by John Stuart Mill. It is meant to establish necessary conditions for a phenomenon. For example, if every instance of an effect E1 (E2, E3, etc.) had in common a particular condition C1, and it is observed that among all cases of E, only C1 and no other conditions (C2, C3, etc.) were common, it can be concluded that C1 is the necessary condition for E. See Mill’s canons.