The third reason is because, since voluntary actions receive their species from the end, as stated above (3), they must needs receive their genus from the last end, which is common to them all: just as natural things are placed in a genus according to a common form. Since, then, all things that can be desired by the will, belong, as such, to one genus, the last end must needs be one. And all the more because in every genus there is one first principle; and the last end has the nature of a first principle, as stated above. Now as the last end of man, simply as man, is to the whole human race, so is the last end of any individual man to that individual. Therefore, just as of all men there is naturally one last end, so the will of an individual man must be fixed on one last end.