(Παναίτιος), son of Nicagoras, descended from a family of long-standing celebrity, was born in the island of Rhodes (Suid. s.v. Strab. xiv. p.968). He is said to have been a pupil of the grammarian Crates, who taught in Pergamum (Strab. xiv. p.993c.), and after that to have betaken himself to Athens, and there attached himself principally to the stoic Diogenes, of Babylon, and his disciple Antipater of Tarsus (Suid. s.v. Cic. de Divin. 1.3). He also availed himself at Athens of the instruction of the learned Periegete Polemo. according to Van Lynden's very probable emendation of the words of Suidas (s. h. v. Comp. Van Lynden, Disputatio Historico-critica de Panaetio Rhodio, Lugd. Batav. 1802, p. 36, &c.). Probably through Laelius, who had attended the instructions, first of the Babylonian Diogenes, and then of Panaetius (Cic. de Fin. 2.8), the latter was introduced to the great P. Scipio Aemilianus, and, like Polybius before him (Suid. s. v. Παναίτιος, comp. s. v. Πολύβιος, and Van Lynden, p. 40, &c.), ggined his friendship (Cic. de Fin. 4.9. de Off 1.26, de Amic. 5. 27, comp. Orat. pro Muren. 31), and acconmpanied him on the embassy which he undertook, two years after the conquest of Carthage, to the kings of Egypt and Asia in alliance with Rome (Vell. 1.13.3; Cic. Aead. 2.2; Plut. Apophth. p. 200e.; comp. Moral. p. 777a.). Panaetius appears to have spent the latter part of his life in Athens, after the death of Antipater, as head of the stoic school (Cic. de Divin. 1.3); at all events he died in Athens (Suid. s. v.), and that before B. C. 111, in which year L. Crassus found there no longer Panaetius himself, but his disciple Mnesarchus (Cic. de Orat. 1.11). Neither the year when Panaetius was born, nor the age attained by him, is stated; all we know is, that he composed the books on Moral Obligations thirty years before his death (Cic. de Off. 3.2, after Posidonius), and that in those books mention was made of Scipio, as it seems, as being already dead (Cic. de Off. 1.26, 2.22). He could scarcely have been much older or younger than Scipio Aemilianus, who died B. C. 129, and was born B. C. 185 (see Van Lynden, 1. c. p. 11, &c. comp. p. 46, &c.). Suidas (s. v.) is the only one who knows anything of an older Panaetius of Rhodes; though in the passage referred to he does not distinguish these two Rhodians of the same name, whom he sets down, from one another. He was probably led to that statement by the erroneous assumption of an ignorant sophist, that Panaetius had been the instructor of the elder Scipio Africanus (Gel. 17.21; comp. Van Lynden, p. 6, &c.)

The principal work of Panaetius was, without doubt, his treatise on the theory of moral obligation (περὶ τοῦ καθήκοντος), composed in three books. In this he proposed to investigate, first, what was moral or immoral; then, what was useful or not useful; and lastly, how the apparent conflict between the moral and the useful was to be decided ; for, as a Stoic, he could only regard this conflict as apparent. The third investigation he had expressly promised at the end of the third book, but had not carried out (Cic. Att. 16.11, de Off. 3.2, 3, comp. 1.3, 3.7, 2.25); and his disciple Posidonius seems to have only timidly (ib. 3.2) and imperfectly supplied what was wanting; at least Cicero, who in his books on Moral Obligations intended, not indeed to translate, but to imitate in his own manner, our Rhodian (ib. 2.17, 3.2, 1.2, ad Att. l.c.), in the third section of the subject, which was not carried out by his guide, did not follow Posidonius, but declares that he had completed independently and without assistance what Panaetius had left untouched (de Off. 3.7). To judge from the insignificant character of the deviations, to which Cicero himself calls attention, as for example, the endeavour to define moral obligation (ib. 1.2), the completion of the imperfect division into three parts (1.3, comp. 2.25), the rejection of unnecessary discussions (2.5), small supplementary additions (2.24, 25), in the first two books Cicero has borrowed the scientific contents of his work from Panaetius, without any essential alterations. The Roman philosopher seems to have been induced to follow Panaetius, passing by earlier attempts of the Stoa to investigate the philosophy of morals, not merely by the superiority of his work in other respects, but especially by the endeavour that prevailed throughout it, laying aside abstract investigations and paradoxical definitions, to exhibit in an impressive manner the philosophy of morals in its application to life (de Off 2.10). Generally speaking, Panaetius, following Aristotle, Xenocrates, Theophrastus, Dicaearchus, and especially Plato, had softened down the harsh severity of the older Stoics, and, without giving up their fundamental definitions, had modified them so as to be capable of being applied to the conduct of life, and clothed them in the garb of eloquence (Cic. de Fin. 4.28, Tuscul. 1.32, de Leg. 3.6; comp. Plut. de Stoic. Repugnant. p. 1033b.; and Van Lynden, p. 60, &100.83, &c.). With him begins the endeavour to supply eclectically the deficiencies in the stoic theory, and to mould it into a new shape; so that among the Neo-Platonists he passed for a Platonist (Proclus, in Plat. Tim. p. 50). For this reason also he assigned the first place in philosophy to physics, not to dialectics (D. L. 7.41), and appears not to have undertaken any original treatment of the latter. In physics he gave up the stoic doctrine of the conflagration of the world (Cic. de Nat. Deor. 2.46, comp. 142; Stobaeus, Ecl. Phys. i. p. 414), endeavoured to simplify the division of the faculties of the soul (Nemes. de Nat. Hom. 100.15; Tertull. de Anima, 100.14), doubted the reality of divination (Cic. de Divin. 1.3, 2.42, 47, Aead. 2.33, comp. Epiphanius, ad v. Haeres. 2.9). In ethics he recognised only a two-fold direction of virtue, the theoretical and the practical, answering to the dianoietic and the ethical of Aristotle (D. L. 7.92); endeavoured to bring the ultimate object of life into nearer relation to natural impulses (ἐκ φίσεως ἀφορμαί ; Clem. Al. Strom. ii. p. 497), and to render manifest by similes the inseparability of the virtues (Stobaeus, Eel. Eth. ii. p. 112); pointed out that the recognition of the moral, as something to be striven after for its own sake, was a leading fundamental idea in the speeches of Demosthenes (Plut. Demosth. p. 852a.); would not admit the harsh doctrine of apathy (A. Gellius, 12.5), and, on the contrary, vindicated the claim of certain pleasurable sensations to be regarded as in accordance with nature (Sext. Empir. ad v. Math, 11.73), while he also insisted that moral definitions should be laid down in such a way that they might be applied by the man who had not yet attained to wisdom (Seneca, Epist. 116). That Cicero has not reproduced the entire contents of the three books of Panaetius, we see from a fragment taken from them, which is not found in Cicero, but has been preserved by A. Gellius (13.27), and which at the same time makes us acquainted with the Rhodian's treatment of his subject in its rhetorical aspects. A similar mode of setting forth his subject, directed to its concrete relations, and rendered intelligible by examples and similes, was to be found, if we may judge by the scanty quotations from it that we have, in his treatise on Equanimity (περὶ εὺθυμίας ; D. L. 9.20, which Plutarch probably had before him in that composition of his which bears the same name), and in those on the Magistrates (Cic. de Legg. 3.5, 6), on Providence (Cic. Att. 13.8), on Divination (see above), and the letter to Q. Aelius Tubero. His work on the philosophical sects (περὶ αἱρέσεων, D. L. 2.87) appears to have been rich in facts and critical remarks (Van Lynden, p. 62, &c.), and the notices which we have about Socrates, and on the books of Plato and others of the Socratic school, given on the authority of Panaetius, were probably taken from that work.