Note 417 — Part I.—Experience

"If we take fifty crucibles of molten matter and let them cool, and fifty solutions and let them evaporate, all will crystallize. Sulphur, sugar, alum, salt—substances, temperatures, circumstances—all are as different as they can be. We find one, and only one, common fact—the change from the liquid to the solid state—and conclude, therefore, that this change is the invariable antecedent of crystallization. Here we have an example of the Method of Agreement. Its canon is:—

"'I. If two or more instances of the phenomenon under investigation have only one circumstance in common, the circumstance in which alone all the instances agree, is the cause (or effect) of the given phenomenon.'"—Ibid. I. 422.