It may be useful to review briefly the argument from definition. The argument from definition, in the sense we shall employ here, includes all arguments from the nature of the thing. Whether the genus is an already recognized convention, or whether it is defined at the moment by the orator, or whether it is left to be inferred from the aggregate of its species, the argument has a single postulate. The postulate is that there exist classes which are determinate and therefore predicable. In the ancient proposition of the schoolroom, “Socrates is mortal,” the class of mortal beings is invoked as a predicable. Whatever is a member of the class will accordingly have the class attributes. This might seem a very easy admission to gain, but it is not so from those who believe that genera are only figments of the imagination and have no self-subsistence. Such persons hold, in the extreme application of their doctrine, that all deduction is unwarranted assumption; or that attributes cannot be transferred by imputation from genus to species. The issue here is very deep, going back to the immemorial quarrel over universals, and we shall not here explore it further than to say that the argument from definition or genus involves a philosophy of being, which has divided and probably will continue to divide mankind. There are those who seem to feel that genera are imprisoning bonds which serve only to hold the mind in confinement. To others, such genera appear the very organon of truth. Without going into that question here, it seems safe to assert that those who believe in the validity of the argument from genus are idealists, roughly, if not very philosophically, defined. The evidence that Lincoln held such belief is overwhelming; it characterizes his thinking from an early age; and the greatest of his utterances (excepting the Gettysburg Address, which is based upon similitude) are chiefly arguments from definition. In most of the questions which concerned him from the time he was a struggling young lawyer until the time when he was charged with the guidance of the nation, Lincoln saw opportunity to argue from the nature of man. In fact, not since the Federalist papers of James Madison had there been in American political life such candid recourse to this term. I shall treat his use of it under the two heads of argument from a concept of human nature and argument from a definition of man. Lincoln came early to the conclusion that human nature is a fixed and knowable thing. Many of his early judgments of policy are based on a theory of what the human being _qua_ human being will do in a given situation. Whether he had arrived at this concept through inductive study—for which he had varied opportunity—or through intuition is, of course, not the question here; our interest is in the reasoning which the concept made possible. It appears a fact that Lincoln trusted in a uniform predictability of human nature.