22. MAN'S NATURE.--Moralists ancient and modern have had a good deal to say about the nature of man. To some of them it has seemed rather a simple thing to describe it. Its constitution, as they have conceived it, has furnished them with certain principles which should guide human action. Aristotle, who assumed that every man seeks his own good, conceived of his good or "well-being" as largely identical with "well- doing." This "well-doing" meant to him "fulfilling the proper functions of man," or in other words acting as the nature of man prescribes. [Footnote: _Politics_, i, 2. See, further, on _Man's Nature_, chapter xxvi.] To the Stoic man's duty was action in accordance with his nature. [Footnote: MARCUS AURELIUS, _Thoughts_, v, 1.] Butler, [Footnote: _Sermons on Human Nature_, ii] many centuries later, found in man's nature a certain "constitution," with conscience naturally supreme and the passions in a position of subordination. This "constitution" plainly indicated to him the conduct appropriate to a human being. Such appeals to man's nature we are apt to listen to with a good deal of sympathy. Manifestly, man differs from the brutes, and they differ, in their kind, from each other. To each kind, a life of a certain sort seems appropriate. The rational being is expected to act rationally, to some degree, at least. In our dealings with creatures on a lower plane, we pitch our expectations much lower. And the behavior we expect from each is that appropriate to its kind. The bee and the ant follow unswervingly their own law, and live their own complicated community life. However the behavior of the brute may vary in the presence of varying conditions, the degree of the variation seems to be determined by rather narrow limits. These we recognize as the limits of the nature of the creature. It dictates to itself, unconsciously, its own law of action, and it follows that law simply and without revolt. When we turn to man, "the crown and glory of the universe," as Darwin calls him, we find him, too, endowed with a certain nature in an analogous sense of the word. He has capacities for which we look in vain elsewhere. The type of conduct we expect of him has its root in these capacities. Human nature can definitely be expected to express itself in a human life,--one lower or higher, but, in every case, distinguishable from the life of the brute. It means something to speak of the physical and mental constitution of man, that mysterious reservoir from which his emotions and actions are supposed to flow. We feel that we have a right to use the expression, even while admitting that the brain of man is, as far as psychology is concerned, almost unexplored territory, and that the relation of mind to brain is, and is long likely to remain, a subject of dispute with philosophers and psychologists.