Those who walked the Stoa Poikile, the Painted Porch, therefore believed they had a duty to something outside their whim and sentiment. The Stoic believed he had a duty, as Kierkegaard would later put it, to the universal. The Stoics taught the doctrine that the divine or eternal plan was reflected in divine law, and that law would be found in the nature of that being. Thus, life in accordance with nature meant conformity to the divine or eternal law and divine will. Ultimately, this conformity with nature resulted in virtue, which led to happiness, as the person would be directed to his true, authentic, and designed good.
As Michael Bertram Crowe puts it in his The Changing Profile of the Natural Law:
In a word, Stoicism is an eclectic system marked by a stern and practical moral bent. The real business of philosophy is the moral conduct of man . . . . Virtue is the primary object of philosophy; ethics, therefore, is the most important branch. . . . Specifically this knowledge is knowledge of the world-order or of the universal law to which the individual must submit himself--and here is the point of insertion of the natural law in the system. We must know nature in order to follow nature's law; the universal law is the law of nature and the Stock morality is epitomised in the maxim: "Live according to nature" (homologoumenos te physei zen).
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) [τέλοςεἶπε τὸ ὁμολογουμένως τῇ φύσει ζῆν], 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 work On End. 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. By the nature with which our life ought to be in accord, Chrysippus understands both universal nature and more particularly the nature of man, whereas Cleanthes takes the nature of the universe alone as that which should be followed, without adding the nature oftheindividual.(R. D. Hicks translation, from http://www.molloy.edu/sophia/seneca/DL_stoicism.htm)
87 Διόπερ πρῶτος ὁ Ζήνων ἐν τῷ Περὶ ἀνθρώπου φύσεως τέλοςεἶπε τὸ ὁμολογουμένως τῇ φύσει ζῆν, ὅπερ ἐστὶ κατ' ἀρετὴν ζῆν·ἄγει γὰρ πρὸς ταύτην ἡμᾶς ἡ φύσις. ὁμοίως δὲ καὶ Κλεάνθης ἐντῷ Περὶ ἡδονῆς καὶ Ποσειδώνιος καὶ Ἑκάτων ἐν τοῖς Περὶ τελῶν(Gomoll 1). πάλιν δ' ἴσον ἐστὶ τὸ κατ' ἀρετὴν ζῆν τῷ κατ'ἐμπειρίαν τῶν φύσει συμβαινόντων ζῆν, ὥς φησι Χρύσιππος ἐν τῷ 88 πρώτῳ Περὶ τελῶν· μέρη γάρ εἰσιν αἱ ἡμέτεραι φύσεις τῆς τοῦὅλου. διόπερ τέλος γίνεται τὸ ἀκολούθως τῇ φύσει ζῆν, ὅπερἐστὶ κατά τε τὴν αὑτοῦ καὶ κατὰ τὴν τῶν ὅλων, οὐδὲν ἐνεργοῦνταςὧν ἀπαγορεύειν εἴωθεν ὁ νόμος ὁ κοινός, ὅσπερ ἐστὶν ὁ ὀρθὸςλόγος, διὰ πάντων ἐρχόμενος, ὁ αὐτὸς ὢν τῷ Διί, καθηγεμόνιτούτῳ τῆς τῶν ὄντων διοικήσεως ὄντι· εἶναι δ' αὐτὸ τοῦτο τὴν τοῦεὐδαίμονος ἀρετὴν καὶ εὔροιαν βίου, ὅταν πάντα πράττηται κατὰτὴν συμφωνίαν τοῦ παρ' ἑκάστῳ δαίμονος πρὸς τὴν τοῦ τῶν ὅλωνδιοικητοῦ βούλησιν. ὁ μὲν οὖν Διογένης τέλος φησὶ ῥητῶς τὸεὐλογιστεῖν ἐν τῇ τῶν κατὰ φύσιν ἐκλογῇ. Ἀρχέδημος δὲ τὸπάντα τὰ καθήκοντα ἐπιτελοῦντα ζῆν. 89 Φύσιν δὲ Χρύσιππος μὲν ἐξακούει, ᾗ ἀκολούθως δεῖ ζῆν, τήντε κοινὴν καὶ ἰδίως τὴν ἀνθρωπίνην· ὁ δὲ Κλεάνθης τὴν κοινὴνμόνην ἐκδέχεται φύσιν, ᾗ ἀκολουθεῖν δεῖ, οὐκέτι δὲ καὶ τὴν ἐπὶμέρους.
(Greek text from http://www.mikrosapoplous.gr/dl/dl07.html)
To be sure, there were problems with the Stoic view both in theory and practice. To name a couple, the Stoic doctrine was pantheistic and advanced some questionable moral values (e.g., suicide, most notably). However, the Stoic concept of the natural law, despite its faults, was not fundamentally unsound, and, much like the Greek to whom St. Paul preached, could be baptized and infused with the spirit of the Gospel. The doctrine of the Stoa Poikile, the Painted Porch, found easy venue in the Ecclesia Christou, the Church of Christ. And so, as Crowe puts it,
The Stoic philosophy, then, has come to articulate the nature law in a new and systematic way, a way that will survive, not only in its phraseology but in a great part of its structure, into the Christian and later formulations of the doctrine. And not alone the explicit teaching on the natural law, but other Stoic moral doctrines, such as that of evident principles, common to all men and prior to experience, principles that include a certain knowledge of good and evil, the elements of the virtues, the recognition of the goodness and eternity of God, will be taken up, corrected and developed by Christians. And so the claim of the Stoics to be the founders of the doctrine of the natural law will be substantiated. This formative influence of Stoicism was transmitted to the Fathers and later to the scholastics by Cicero and by the Roman law.
(Michael Betram Crowe, The Changing Profile of the Natural Law (Martinus Nijhoff: The Hague, 1977), 30-36.
The Ecclesia Christou, the Church of Christ