features of the Stoic ethics may be simply deduced from their one fundamental notion, that rational action or virtue is the only good. [(2) Scientific side of the Stoic system.] Not only does this view of ethics require a peculiar theory of the world to serve as its scientific basis, but it has a reflex action also, influencing alike the tone and the results of theoretic enquiry. If the duty of man is declared to consist in bringing his actions into harmony with the laws of the universe, it becomes also necessary that he should endeavour himself to know the world and its laws. The more his knowledge of the world increases, the greater will be the value which he attaches to the forms of scientific procedure. If, moreover, man is required to be nothing more than an instrument of the universal law, it is only consistent to suppose an absolute regularity of procedure in the universe, an unbroken connection of cause and effect, and ultimately to refer everything to one highest all-moving cause, and to include everything under one primary substance. If in human life the individual has no rights as against the laws of the universe, then all that is of individual occurrence in the world is powerless against universal necessity. On the other hand, if in the case of man everything turns upon strength of will, then likewise in the universe the acting power must be regarded as the highest and most exalted. There arises thus that view of the world as a series of forces which constitutes one of the most peculiar and thorough-going characteristics of the Stoic view of nature. [1032] Lastly, if such excessive importance is attached to practical conduct as is done by the Stoics, that sensuous view of the world which finds its crudest expression in the Stoic Materialism and reliance on the senses, [1033] will most nearly accord with speculation. At the same time the Materialism of the Stoics is limited and corrected by the conception of the universe and of a divine all-penetrating power and reason, just as their appeal to the senses is by the demand for the formation of conceptions, and the general application of the process of demonstration; the truth of knowledge itself is based on a practical postulate, the greater or less certainty of which is measured by the strength of personal conviction. If these elements proved too contradictory to be harmonised; if the Materialism of the Stoics was at variance with their view of the world as a series of forces; if appeals to the senses were obviously in conflict with logical method, it was at least thereby clearly established that a practical and not a purely intellectual interest lay at the root of their system. [(3) Connection of the moral and scientific elements.]