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38 even those who insisted most emphatically on the invariable necessity of the natural order—as, e.g., Empedocles, Plato, and Aristotle—did not designate this by the term law. With very few exceptions, before the time of the Stoics this word was applied exclusively to norms of human conduct, the laws of nature, when this expression was used, meaning such rules of conduct as were common to all men and binding upon them by virtue of their very nature. "It was the founder of the Stoic school," says Zeller, "who first brought into common use the concept of law as applied to the natural order of things." The extension of law from the sphere of human action to the physical world was a natural consequence of the fundamental doctrine of Stoicism. The Stoics believed in an ultimate ground and cause of the world, which was not merely the material substance of things, but was at the same time creative Reason. The natural order and necessity in the universe they regarded as the expression of the will of that ultimate Reason, and hence called it the law of nature. As man and nature are both under the same divine law-giver, no distinction was made between natural law and moral law. In the absence of scientific precision the same confusion prevailed throughout the Middle Ages. The laws that determine the order of nature and those which express the duty of man were regarded alike as divine commands. It is only since the sixteenth and seventeenth centuries that philosophers and men of science have held a clear conception of natural law as the expression of the uniformities of the phenomenal world, in distinction from the primary use of law as applied to norms of human conduct.

Were we tracing the history of the concept of law in physical science, we should have now to consider what use the Stoics made of this law of nature in explaining the material world; our interest here, however, is in the use they made of the concept in their moral philosophy.