The case may be made clearer by considering a genuinely-empirical generalization, such as “All men are mortal.” It is plain that we believe this proposition, in the first place, because there is no known instance of men living beyond a certain age, and in the second place because there seem to be physiological grounds for thinking that an organism such as a man’s body must sooner or later wear out. Neglecting the second ground, and considering merely our experience of men’s mortality, it is plain that we should not be content with one quite clearly understood instance of a man dying, whereas, in the case of “two and two are four,” one instance does suffice, when carefully considered, to persuade us that the same must happen in any other instance. Also we can be forced to admit, on reflection, that there may be some doubt, however slight, as to whether all