 Since the Stoics stress the systematic nature of their philosophy, the ideal way to evaluate the Stoics’ distinctive ethical views would be to study them within the context of a full exposition of their philosophy. Here, however, we meet with the problem about the sources of our knowledge about Stoicism. We do not possess a single complete work by any of the first three heads of the Stoic school: the ‘founder,’ Zeno of Citium in Cyprus (344–262 BCE), Cleanthes (d. 232 BCE) or Chrysippus (d. ca. 206 BCE). Chrysippus was particularly prolific, composing over 165 works, but we have only fragments of his works. The only complete works by Stoic philosophers that we possess are those by writers of Imperial times, Seneca (4 BCE–65 CE), Epictetus (c. 55–135) and the Emperor Marcus Aurelius (121–180) and these works are principally focused on ethics. They tend to be long on moral exhortation but give only clues to the theoretical bases of the moral system. For detailed information about the Old Stoa (i.e. the first three heads of the school and their pupils and associates) we have to depend on either doxographies, like pseudo-Plutarch Philosophers’ Opinions on Nature, Diogenes Laertius’ Lives of Eminent Philosophers (3rd c. CE), and Stobaeus’ Excerpts (5th c. CE) – and their sources Aetius (ca. 1st c. CE) and Arius Didymus (1st c. BCE–CE) – or other philosophers (or Christian apologists) who discuss the Stoics for their own purposes. Nearly all of the latter group are hostile witnesses. Among them are the Aristotelian commentator Alexander of Aphrodisias (late 2nd c. CE) who criticises the Stoics in On Mixture and On Fate, among other works; the Platonist Plutarch of Chaeronea (1st-2nd c. CE) who authored works such as On Stoic Self-Contradictions and Against the Stoics on Common Conceptions; the medical writer Galen (2nd c. CE), whose outlook is roughly Platonist; the Pyrrhonian skeptic, Sextus Empiricus (2nd c. CE); Plotinus (3rd c. CE); the Christian bishops Eusebius (3rd–4th c. CE) and Nemesius (ca. 400 CE); and the sixth-century neoplatonist commentator on Aristotle, Simplicius. Another important source is Cicero (1st c. BCE). Though his own philosophical position derives from that of his teacher Philo of Larissa and the New Academy, he is not without sympathy for what he sees as the high moral tone of Stoicism. In works like his Academic Books, On the Nature of the Gods, and On Ends he provides summaries in Latin, with critical discussion, of the views of the major Hellenistic schools of thought. 