Theophrastus of Eresos (Lesbos) was first a pupil of Plato, then of Aristotle, whose confidence he won to such a degree that in his will Aristotle appointed him guardian of his children and his successor at the Lyceum. After Aristotle's death (322 BCE), Theophrastus took over the school and was its scholarch for thirty-five years. Under him the Lyceum reached its largest size — Diogenes Laertius (V 37) reports an attendance of two thousand, an unprecedented figure for an ancient philosophical school.

He was the author of some 240 works (Diogenes Laertius preserves the full catalogue at V 42–50) — on logic, physics, natural science (especially botany and mineralogy), psychology, ethics, rhetoric, political theory, physiognomy, and the history of philosophy. Only three works survive entire: the Characters (ethical sketches of thirty human types — the paragraph on the flatterer, the paragraph on the superstitious man, and so on); the Enquiry into Plants (Περὶ φυτῶν ἱστορία); and On the Causes of Plants (Περὶ φυτῶν αἰτιῶν). With these two books of botany Theophrastus founded the European science of botany. The remaining works survive in fragments, often through citation by later authors.

Theophrastus is the systematiser and developer of the Aristotelian tradition. His chief contributions:

• Logic. He developed modal syllogistic, introduced the notion of the hypothetical syllogism, and worked out the doctrine of the topoi. Through this work Peripatetic logic passed down to the Middle Ages and to the Stoics.
• Natural science. He founded botany as an independent science and wrote (now lost) works on mineralogy, meteorology, and the physiology of the senses.
• Ethics. He developed Aristotle's teaching of the virtues as mean-dispositions; he introduced fine distinctions between types of offence, especially under the criteria of voluntariness (ἑκούσιον / ἀκούσιον) and motivating force (the passion that drives an act). This work largely prepared the ground for the Stoic doctrine of the passions (see below on Chrysippus).
• History of philosophy. Φυσικῶν δόξαι — the first systematic doxography of the Presocratics; the source of much of what we know about early Greek philosophy.

Influence on the Stoics: Chrysippus, in shaping his theory of the passions, built largely on Theophrastus' typology. The very strategy of Chrysippus — defining the passions through false assents — is close to the Peripatetic classification of offences by motivating force.

• 02-10 — the only sustained citation in Marcus. From a lost ethical work: the thesis that offences born of ἐπιθυμία (desire) are graver than offences born of θυμός (anger), because they are initiated out of pleasure and one's own impulse, and not driven by a preceding pain. Marcus cites it approvingly: «ὀρθῶς οὖν καὶ φιλοσοφίας ἀξίως» — "rightly, and worthily of philosophy." This is the only place in the whole of Book II where Marcus names a philosophical authority.

• Diogenes Laertius V 36–57 — biography
• W. W. Fortenbaugh, P. M. Huby, R. W. Sharples, D. Gutas, Theophrastus of Eresus: Sources for his Life, Writings, Thought and Influence, Brill, 1992–2005 (the standard collection of fragments, 10 volumes)
• W. W. Fortenbaugh, Theophrastus of Eresus: Commentary Volume 6.1, Sources on Ethics, Brill, 2011
• P. Huby, Theophrastus of Eresus: Commentary Volume 4, Psychology, Brill, 1999
• F. Wehrli, ed., Die Schule des Aristoteles, vol. I (the Theophrastean texts as edited before Fortenbaugh)