All things are parts of one single system, which is called Nature; the individual life is good when it is in harmony with Nature. In one sense, every life is in harmony with Nature, since it is such as Nature’s laws have caused it to be; but in another sense a human life is only in harmony with Nature when the individual will is directed to ends which are among those of Nature. Virtue consists in a will which is in agreement with Nature. (Zeno of Citium, ~ 300B.C.)