Wisdom from the Early Stoics, Zeno of Citium 19

I have decided to give a general account of all the Stoic doctrines in the life of Zeno because he was the founder of the School. . . . 

Philosophic doctrine, say the Stoics, falls into three parts: one physical, another ethical, and the third logical. 

Zeno of Citium was the first to make this division in his Exposition of Doctrine, and Chrysippus too did so in the first book of his Exposition of Doctrine and the first book of his Physics; and so too Apollodorus and Syllus in the first part of their Introductions to Stoic Doctrine, as also Eudromus in his Elementary Treatise on Ethics, Diogenes the Babylonian, and Posidonius.

These parts are called by Apollodorus "Heads of Commonplace"; by Chrysippus and Eudromus specific divisions; by others generic divisions. 

Philosophy, they say, is like an animal, Logic corresponding to the bones and sinews, Ethics to the fleshy parts, Physics to the soul. 

Another simile they use is that of an egg: the shell is Logic, next comes the white, Ethics, and the yolk in the center is Physics. 

Or, again, they liken Philosophy to a fertile field: Logic being the encircling fence, Ethics the crop, Physics the soil or the trees. 

Or, again, to a city strongly walled and governed by reason. 
 
No single part, some Stoics declare, is independent of any other part, but all blend together. Nor was it usual to teach them separately. 

Others, however, start their course with Logic, go on to Physics, and finish with Ethics; and among those who so do are Zeno in his treatise On Exposition, Chrysippus, Archedemus and Eudromus.

—Diogenes Laërtius, 7.38-40