Page:Russell - The Problems of Philosophy, 1912.djvu/170

166 priori knowledge is seen to have been based upon an error.

It will serve to make the point clearer if we contrast our genuine a priori judgment with an empirical generalisation, such as "all men are mortals." Here as before, we can understand what the proposition means as soon as we understand the universals involved, namely man and mortal. It is obviously unnecessary to have an individual acquaintance with the whole human race in order to understand what our proposition means. Thus the difference between an a priori general proposition and an empirical generalisation does not come in the meaning of the proposition; it comes in the nature of the evidence for it. In the empirical case, the evidence consists in the particular instances. We believe that all men are mortal because we know that there are innumerable instances of men dying, and no instances of their living beyond a certain age. We do not believe it because we see a connection between the universal man and the universal mortal. It is true that if physiology can prove,