The will to power, the principle which Nietzsche held to be the elementary expression of life, must be understood in order for one to comprehend the Nietzschean system of ethics. Throughout all the books which followed "The Joyful Wisdom" we have indirect references to it and conclusions based on its assumption as a hypothesis. And, although it was never definitely and finally defined until the publication of the notes comprising "The Will to Power," it nevertheless was the actuating motive in all Nietzsche's constructive writings. Simply stated, the will to power is the biological instinct to maintenance, persistence and development. Nietzsche holds that Darwin's universal law of the instinct to mere survival is a misinterpretation of the forces at work in life. He points out that existence is a condition--a medium of action--and by no means an end. It is true that only the fittest survive in nature as a result of the tendency to exist; but this theory does not account for the activities which take place after existence has been assured. In order to explain these activities Nietzsche advances the theory of the will to power and tests all actions by it. It will be seen that by this theory the universal law of Darwin is by no means abrogated, but rather is it explained and developed. In the operation of Darwin's biological law there are many forces at work. That is to say, once the fact of existence is established, numerous forces can be found at work within the limits of existence. We know that the forces of nature--acting within the medium of existence which is an _a priori_ condition--are rarely unified and directed toward the same result. In short, they are not reciprocal. To the contrary, they work more often against each other--they are antagonistic. Immediately a war of forces takes place; and it is this war that constitutes all action in nature. A force in nature directed at another force calls forth a resistance and counter-force; and this instinct to act and to resist is in itself a will to act. Otherwise, inertia would be the condition of life, once mere existence was assured by the fittest. But life is not inert. Even when certain organisms have accomplished the victory for existence, and are no longer moved by a necessity to struggle for mere being, the will to action persists; and this will to action, according to Nietzsche, is the will for power, for in every clash of forces, there is an attempt on the part of each force to overcome and resist the antagonistic one. The greater the action, the greater the antagonism. Hence, this tendency in all forces to _persist_ is at bottom a tendency of self-assertion, of overcoming counter-forces, of augmenting individual power. Wherever this will to persist is found, Nietzsche argues that the will to act is present; and there can be no will to act without a will to power, because the very desire for existence and development is a desire for power.