Other illustrations might be adduced, but these are sufficient for our purpose. The truth is, there is not a phenomenon known to man that can properly be said to be the result of the action of _one_ invariable and universal force, not even the falling of a stone to the earth; for some force must have previously been exerted to raise the stone from the earth, which force is represented by energy of position, or "potential energy."[241] And this potential energy is the exact numerical equivalent of the energy of motion which it acquires in falling--_i. e._, the mass multiplied by the square of the velocity. Every event, every change in nature, is due to "some _variable_ combinations of invariable forces."[242] Material causes are always complex. Every law of nature is liable to counteraction and modification by other laws, and the most fundamental fact of the universe is that material forces are adjusted, combined, and modified in endless modes in order to the fulfillment of purposes and ends. The phenomena of life present a vast series of such adjustments and modifications. The mechanical and chemical forces are controlled and subordinated by the vital force, so that life has been defined as "a _resistance_ to the physical forces of matter"[243]--a resistance which Liebig regards as in a certain degree invincible. Living matter is the seat of energy, and so long as it is living, can overcome the primary law of the _inertia_ of matter, and moves spontaneously.[244] Living matter overcomes the attraction of gravitation, and resists, suspends, and modifies the action of chemical affinity.[245] It is in direct opposition to chemical affinity that organized beings exist. Thus the various forms of energy are mutually conditioned. The mechanical, chemical, and electrical energies are counteracted by the vital force. And all the forces and energies of nature are controlled and subordinated by a higher force which orders means to ends, and adapts structure to function, viz., an _Intelligent Will_. The conviction finally becomes irresistible that nature is a system of things designed to be subject to Mind, and that a law of design is the highest law of the universe. It must now be obvious that we can reach no definite conclusion in regard to the question under discussion--the uniformity of nature--unless we have a clear and precise conception of the meaning of the term "nature." The word is employed, even by men of science, in a very loose and ambiguous sense. At one time it is used to denote the totality of sensible phenomena; at another, the conditions or causes of phenomena; again, the relations of phenomena; and often, all these collectively. We must endeavor to extricate ourselves from this confusion.