Note 420 — Part I.—Experience

"Let us take two facts—as the presence of the earth and the oscillation of the pendulum; or, again, the presence of the moon and the flow of the tide. To connect these phenomena directly, we should have to suppress the first of them, and see if this suppression would occasion the stoppage of the second. Now, in both instances, such suppression is impossible. So we employ an indirect means of connecting the phenomena. We observe that all the variations of the one correspond to certain variations of the other; that all the oscillations of the pendulum correspond to certain different positions of the earth; that all states of the tide correspond to positions of the moon. From this we conclude that the second fact is the antecedent of the first. These are examples of the Method of Concomitant Variations. Its canon is:—

"'V. Whatever phenomenon varies in any manner whenever another phenomenon varies in some particular manner, is either a cause or an effect of that phenomenon, or is connected with it through some fact of causation.'"—Mill's "Logic," I. 435.