In its application therefore to physical science, it may with strict propriety be used to denote an invariable order of events: and if the human analogy could hold in physics it might be used to include the power which originated and enforced them. But as the consideration of will or purpose forms no portion of strictly physical science, and is expressly excluded from it, the term law as used by it ought to denote the invariable order of sequences, and not to include the forces which generate them. Unless this distinction is carefully observed, we shall be in danger of introducing into our reasonings human analogies to which there is nothing corresponding in nature viewed as a mere body of unintelligent forces. The use of the term “law” in physical science ought to be confined to denote the invariable sequences of the material phenomena. Physicists profess to know nothing of efficient causation; or of a lawgiver standing outside his laws and possessing power to enforce them. The whole question of intelligent agency or purpose lies in a region outside their province. Law, as far as physical science is acquainted with it, can consist only of a set of antecedents, followed by an invariable set of consequents. Of any inherent efficacy in these antecedents to produce their consequents, it can affirm nothing. A very popular philosophy even denies the power of the human mind to penetrate beyond this, and affirms that its entire knowledge is limited to phenomena. But physical science also deals with forces. These, and not its laws, are its true principles of causation. Mere invariable sequences can effect nothing; but forces, such as gravitation, heat, electricity, and the entire body of chemical forces, or whatever force they may ultimately be resolved into, can effect much. They are in fact the antecedents of which the invariable order of events are the consequents. Respecting the ultimate principle of force, or what is its real nature, or how it is directed, or came to be, physical science is silent. All that it can do is to observe the order of their occurrence, measure their quantities, and tabulate their results. By this means it rises to the conception of what are called the laws of nature.