The Benthamites, by straining the meaning of words, tried to classify such motives as instinctive impulse, ancient tradition, habit, or personal and racial idiosyncrasy as being forms of pleasure and pain.

But they failed; and the search for a basis of valid political reasoning has to begin again, among a generation more conscious than were Bentham and his disciples of the complexity of the problem, and less confident of absolute success.
In that search one thing at least is becoming clear.

We must aim at finding as many relevant and measurable facts about human nature as possible, and we must attempt to make all of them serviceable in political reasoning.

In collecting, that is to say, the material for a political science, we must adopt the method of the biologist, who tries to discover how many common qualities can be observed and measured in a group of related beings, rather than that of the physicist, who constructs, or used to construct, a science out of a single quality common to the whole material world.
The facts when collected must, because they are many, be arranged.

I believe that it would be found convenient by the political student to arrange them under three main heads: descriptive facts as to the human type; quantitative facts as to inherited variations from that type observed either in individuals or groups of individuals; and facts, both quantitative and descriptive, as to the environment into which men are born, and the observed effect of that environment upon their political actions and impulses.
A medical student already attempts to master as many as possible of those facts about the human type that are relevant to his science.