To see this principle in application, let us take three of the chief sources of argument recognized by the classical rhetoricians. We may look first at the source which is _genus_. All arguments made through genus are arguments based on the nature of the thing which is said to constitute the genus. What the argument from genus then says is that “generic” classes have a nature which can be predicated of their species. Thus _man_ has a nature including _mortality_, which quality can therefore be predicated of the man Socrates and the man John Smith. The underlying postulate here, that things have a nature, is of course a disputable view of the world, for it involves the acceptance of a realm of essence. Yet anyone who uses such source of argument is committed to this wider assumption. Now it follows that those who habitually argue from genus are in their personal philosophy idealists. To them the idea of genus is a reflection of existence. We are saying, accordingly, that arguments which make predominant use of genus have an aspect through this source, and that the aspect may be employed to distinguish the philosophy of the author. It will be found, to cite a concrete example, that John Henry Newman regularly argues from genus; he begins with the nature of the thing and then makes the application. The question of what a university is like is answered by applying the idea of a university. The question of what man ought to study is answered by working out a conception of the nature of man. And we shall find in a succeeding essay that Abraham Lincoln, although he has become a patron for liberals and pragmatists, was a consistent user of the argument from genus. His refusal to hedge on the principle of slavery is referable to a fixed concept of the nature of man. This, then, will serve to characterize the argument from genus. Another important source of argument is _similitude_. Whereas those who argue from genus argue from a fixed class, those who argue from similitude invoke essential (though not exhaustive) correspondences. If one were to say, for example, that whatever has the divine attribute of reason is likely to have also the divine attribute of immortality, one would be using similitude to establish a probability. Thinkers of the analogical sort use this argument chiefly. If required to characterize the outlook it implies, we should say that it expresses belief in a oneness of the world, which causes all correspondence to have probative value. Proponents of this view tend to look toward some final, transcendental unity, and as we might expect, this type of argument is used widely by poets and religionists.[26] John Bunyan used it constantly; so did Emerson.