Three main forces are usually mentioned:  Civilization, Commerce, Christianity.

Before considering these it may be well to note Newton’s first law of motion, that every body will continue in a state of rest or of uniform motion in a straight line unless acted on by some external force; for though this law was affirmed of material bodies, yet its applicability to large groups of men is striking and suggestive.  Not only do human beings have the physical attributes of weight and inertia like other material bodies, but their mental organism, while of a higher order than the physical, is as powerfully affected by external forces.  And though it is true that psychology has not yet secured her Newton, and that no one has yet formulated a law that expresses exactly the action of the minds and spirits of men under the influence of certain mental and moral stimuli or forces, yet we know that our minds and spirits are influenced by fear, hope, ambition, hate, and so forth, in ways that are fairly well understood and toward results that often can be predicted in advance.

Our whole theory of government and our laws of business and every-day life are founded on the belief that men are the same to-day as they were yesterday, and that they will be the same to-morrow.  The whole science of psychology is based on the observed and recorded actions of the human organism under the influence of certain external stimuli or forces, and starts from the assumption that this organism has definite and permanent characteristics.  If this is not so—­if the behavior of men in the past has not been governed by actual laws which will also govern their behavior in the future—­then our laws of government are built on error, and the teachings of psychology are foolish.

This does not mean that any man will necessarily act in the same way to-morrow as he did yesterday, when subjected to the influence of the same threat, inducement, or temptation; because, without grappling the thorny question of free will, we realize that a man’s action is never the result of only one stimulus and motive, but is the resultant of many; and we have no reason to expect that he will act in the same way when subjected to the same stimulus, unless we know that the internal and external conditions pertaining to him are also the same.  Furthermore, even if we cannot predict what a certain individual will do, when exposed to a certain external influence, because of some differences in his mental and physical condition, on one occasion in comparison with another, yet when we consider large groups of men, we know that individual peculiarities, permanent and temporary, balance each other in great measure; that the average condition of a group of men is less changeable than that of one man, and that the degree of permanency of condition increases with the number of men in the group.  From this we may reasonably conclude that, if we know the character of a man—­or a group of men—­and