The teachings of earlier thinkers appear frequently in Plato’s dialogues, sometimes in a verbatim quotation but more often as a paraphrase or loose characterization.1 Plato’s characters endorse the doctrines of Parmenides and the Pythagoreans but criticize the approach taken by the earlier ‘inquirers into nature’—the Ionian thinkers we today consider the founders of Western science and philosophy. In the Phaedo Socrates commends Anaxagoras for identifying mind as the cause of all things but faults him for failing to stick with this hypothesis and resorting instead to material causes. In other dialogues various Presocratics come under fire for failing to recognize the priority of the soul over body and not believing that a divine intelligence arranges all things for the best.
Thales is the only Milesian philosopher-scientist Plato refers to by name, although the Phaedo mentions two doctrines sometimes credited to Anaximenes.2 Plato’s Thales is the familiar combination of scientific inquirer (Theaetetus 174a), inventor (Republic X 600a), and practical sage (Letter II 311a, Hippias Major 281c, Protagoras 343a). Plato also credits Thales with the view that ‘all things are full of gods’ (Laws X 899b), which may explain why he does not mention Thales when he criticizes materialist cosmologies in the Epinomis (988b) and Laws (X 886e). The story of Thales and the Thracian serving girl (Theaetetus 174a) suggests that Plato saw him as a prototype of the philosopher whose theoretical inquiries expose him to public ridicule.
Xenophanes of Colophon
Plato mentions Xenophanes (at Sophist 242d), as one of those who early on spoke of ‘all things as one’, but the two thinkers had more in common than Plato admitted. The proposal to censor poetic depictions of divine misconduct put forward in Republic X echoes sentiments Xenophanes had expressed in B 1, 11, 12, and 22; the call at Apology 36e and Republic V 465d to honor wise civic counselors more than victorious athletes tracks the language of Xenophanes B 3; and the distinction between knowledge and true opinion (endorsed at Meno 98a and elsewhere) appeared in Xenophanes B 34. Plato’s unwillingness to acknowledge his debts to Xenophanes may be due to the fact that Xenophanes endorsed the kind of materialism (cf. B 27 and 29) the Athenian of Laws X claims warrants five years of solitary confinement—and possibly the death penalty. Plato might also have been unhappy with Xenophanes’ assertions that people in different regions conceive of the gods in different terms (Laws X, 889e) and that the one greatest god ‘shakes all things by the thought of his mind’ (B 25) but is otherwise uninvolved in human affairs.
Pythagoras of Samos
The name ‘Pythagoras’ appears just once in the dialogues when (at Republic 600a) Plato describes him as ‘the founder of a way of life.’ But at Philebus 16d Plato alludes to a ‘Prometheus like figure’ who taught that ‘all things consist of a one and many, and have in their nature a conjunction of limit and unlimited’ and that ‘we must go from one form to look for two, if the case admits of there being two, otherwise for three or some other number of forms.' Although the body of evidence relating to ancient Pythagoreanism is much contested, it seems certain that at some point Plato became aware of the mathematical way of thinking championed by contemporary Pythagoreans and embraced it as a fundamental aspect of his own philosophy. Among other things, Plato chose to place his most extensive account of the physical cosmos in the mouth of Timaeus, an imaginary Pythagorean statesman and scientist from southern Italy. In the Republic Plato described the study of mathematics as the best kind of preparation for philosophical dialectic and an essential component in the training of philosopher kings and queens (536d). Plato’s tripartite account of the three parts of the soul bears a close resemblance to Pythagoras’ description of the three kinds of lives (Diogenes Laertius, Lives of the Philosophers VIII, 8). The famous simile of the divided line embodies the same four-fold progression the followers of Pythagoras identified as the sacred tetractys passed down to their generation from the master (Aëtius, I, 3,8). Plato’s definition of justice as ‘psychic harmony’ draws on a conception of musical harmony expressed by the Pythagorean thinker Philolaus of Croton (DK 44 B 6). These and other points of contact fully warrant the conclusion that Plato knew and embraced many of the Pythagorean doctrines of his own era.3
Heraclitus of Ephesus
Plato attributed to Heraclitus the erroneous (indeed, self-defeating) view that all things are changing in all respects all the time (Cratylus 401d, 402a, 411b; Theaetetus 152e, 160d, 177c; and Philebus 43a).4 Plato also expressed contempt for Heraclitus’ aphorism-spouting followers (Theaetetus 180) and, at least according to the character Eryximachus in the Symposium (187a), accused Heraclitus of propounding a doctrine of opposites he himself did not understand. But there is a second, more positive side to this story. On Plato’s view, the thesis of universal change holds true for things located in the physical realm, including human beings (cf. Symposium 207d). Although Eryximachus claims that ‘disharmony in harmony’ can only mean the alternative presence of opposing states, the Stranger at Sophist 242d has no difficulty in attributing to a ‘certain Ionian muse’ the view that ‘the real is both many and one and is held together by enmity and friendship. In parting asunder it is always being drawn together.’ In addition, at Phaedo 65a and Republic 508 Plato indicts the senses as unsuitable sources of knowledge just as Heraclitus had indicted ‘eyes and ears’ as ‘bad witnesses for those with uncomprehending souls’ (B 107). The contrast of sleeping with waking, a leitmotif in the surviving Heraclitus fragments, was also one of Plato’s pet themes. Both philosophers saw humankind as oblivious to the true nature of reality and sought ways to provoke or ‘summon’ the minds of their audiences to gain a more adequate grasp of the realities.
Parmenides of Elea
Parmenides provided Plato with both the wording and philosophical foundation for two key doctrines in his philosophy: his dualistic metaphysics and rationalist theory of knowledge (cf. Diotima’s description of Beauty Itself at Symposium 211a, the linking of knowledge with being at Republic 476, and the denigration of sense perception in the simile of the divided line in Republic VI). On the Parmenidean-Platonic view, ‘what is’ is incapable of ‘not being’ of any kind—and is therefore never changing, moving, being divided, coming into being, or being destroyed. In addition, since knowledge is infallible, it must have as its objects things that remain forever in possession of their attributes, and these (Forms or Ideas) can be apprehended only in thought.5
Empedocles of Akragas
Plato refers frequently to doctrines associated with Empedocles, but subscribed to few of them. At Meno 76c Socrates draws on an Empedoclean theory of ‘effluences’ to define color, but disparages the resulting definition as ‘pompous’. At Theaetetus 152e Socrates includes Empedocles among the earlier thinkers who (mistakenly) held that all things are in the process of becoming. Phaedo 96b contains what is probably a reference to Empedocles’ view of blood as the medium of thought (A 30, 86, and 97); the Timaeus 48b introduces a geometrical improvement on the Empedoclean doctrine of ‘roots’ (B 6 and A 30); and the Sophist alludes to ‘a certain muse in Sicily’ who held that ‘the real is both many and one and held together by enmity and friendship’ (242d, 243a; cf. B 26). But Plato did share Empedocles’ conception of philosophy as both a theoretical inquiry and a practical guide to life, as well as his view of the present life as merely one stage in the soul’s long journey (B 111 and 115 and the myth of Er in Republic X).
Anaxagoras of Clazomenae
Plato credits Anaxagoras with four main views: that the sun and moon are not gods but merely stone and earth (Apology 26d), that the moon receives its light from the sun (Cratylus 409b), that in some sense ‘all things are mingled together’ (Phaedo 72c and Gorgias 465d), and that ‘mind (nous) produces order and is the cause of everything’ (Phaedo 98b, Cratylus 413c and 400a, Laws X, 886d and XII, 967b) But in the Phaedo Socrates faults Anaxagoras for failing to stick to this hypothesis and resorting to causes such as ‘air, aether, water, and many other absurdities’ (98c).
Leucippus and Democritus
One measure of Plato’s hostility toward materialist cosmologies is the fact that the names of the authors of the ancient atomic theory appear nowhere in his writings. That reality should consist entirely of material bodies jostling about in empty space and that events can be fully explained in terms of material causes, were ideas anathema to Plato. Thus when (at Timaeus 53 ff.) he presents his own theory of elements it is so heavily infused with aesthetic, moral, and mathematical considerations as to be hardly recognizable as a form of atomism at all.
1 Other ancient sources tell us little about Plato’s contacts with earlier thinkers. The most detailed account, Diogenes Laertius’ Lives of Eminent Philosophers (Secs. 6, 9, and 10), reports that Plato knew Epicharmus (a philosopher-poet), Cratylus (a follower of Heraclitus), Hermogenes (a follower of Parmenides), and Philolaus and Eurytus (both followers of Pythagoras). Aristotle reports that as a youth Plato ‘became acquainted with Cratylus and the Heraclitean doctrines’ (Metaphysics I. 6, 987a32), and derived some of his ideas from the Pythagoreans. For a critical evaluation of these and other ancient sources, see A. S. Riginos, Platonica: The Anecdotes concerning the Life and Writings of Plato (Brill: Leiden, 1976).
2 The first is a particle-based account of growth (96c-d) and the second a view of the earth as a platter supported by a column of air (99b). Hippolytus credits the latter view to Anaxagoras (DK 59A42), but Aristotle (DK 13A 20) attributes it to Anaximenes, Anaxagoras, and Democritus.
3 For a more detailed account of Plato’s connections with Pythagorean thought see the article by Carl Huffman elsewhere in this volume.
4 In recent years a number of scholars (following the lead of Geoffrey Kirk) have challenged Plato’s attribution of the thesis of ‘radical change’ to Heraclitus. The ‘river fragments’ usually considered to be the basis for Plato’s interpretation can also be read as making rather different points (e.g. the unity of the opposites, and the measured nature of all change). In any case there was at least one aspect of Heraclitean reality that was exempt from change, namely the logos that ‘hold forever’ (B 1).
5 For additional discussion, see the article on Eleatic thought by Herbert Granger elsewhere in this volume.