Wisdom from the Early Stoics, Zeno of Citium 56

This is why Zeno was the first (in his treatise On the Nature of Man) to designate as the end "life in agreement with nature" (or living agreeably to nature), which is the same as a virtuous life, virtue being the goal towards which nature guides us. 

So too Cleanthes in his treatise On Pleasure, as also Posidonius, and Hecato in his work On Ends. 

Again, living virtuously is equivalent to living in accordance with experience of the actual course of nature, as Chrysippus says in the first book of his De Finibus; for our individual natures are parts of the Nature of the whole Universe. 

And this is why the end may be defined as life in accordance with nature, or, in other words, in accordance with our own human nature as well as that of the Universe, a life in which we refrain from every action forbidden by the law common to all things, that is to say, the right reason which pervades all things, and is identical with this Zeus, lord and ruler of all that is. 

And this very thing constitutes the virtue of the happy man and the smooth current of life, when all actions promote the harmony of the spirit dwelling in the individual man with the will of him who orders the Universe. 

Diogenes then expressly declares the end to be to act with good reason in the selection of what is natural. 

Archedemus says the end is to live in the performance of all befitting actions. 

—Diogenes Laërtius, 7.87-88