2. Of course it follows that success in the production of wealth under this modern system depends primarily on the correctness with which men learn the character of economic forces and of the laws under which those forces act. This is the field of the science of political economy, and it is the reason why it is a science. It investigates the laws of forces which are natural, not arbitrary, artificial, or conventional. Some communities have developed a great hatred for persons who held different religious opinions from themselves. Such a feeling would be a great social force, but it would be arbitrary and artificial. Many communities have held that all labor, not mental, was slavish and degrading. This notion, too, was conventional, but it was a great social force where it existed. Such notions, either past or present, are worth studying for historical interest and instruction, but they do not afford the basis for a science whose object is to find out what is true in regard to the relations of man to the world in which he lives. The study of them throws a valuable sidelight on the true relations of human life, just as the study of error always throws a sidelight upon the truth, but they have no similarity to the law that men want the maximum of satisfaction for the minimum of effort, or to the law of the diminishing return from land, or to the law of population, or to the law of supply and demand. Nothing can be gained, therefore, by mixing up history and science, valuable as one is to the other. If men try to carry on any operation without an intelligent theory of the forces with which they are dealing, they inevitably become the victims of the operation, not its masters. Hence they always do try to form some theory of the forces in question and to plan the means to the end accordingly. The forces of nature go on and are true only to themselves. They never swerve out of pity for innocent error or well-intentioned mistakes. This is as true of economic forces as of any others. What is meant by a good or a bad investment, except that one is based on a correct judgment of forces and the other on incorrect judgment? How would sagacity, care, good judgment, and prudence meet their reward if the economic forces swerved out of pity for error? We know that there is no such thing in the order of nature.