The Philosophies That Shaped Plato’s Thought

Five traditions shaped Plato’s thinking more than any other: the questioning of Socrates, the Heraclitean doctrine of flux, the Eleatic arguments of Parmenides, Pythagorean mathematics and religion, and the rhetoric of the Sophists, whom he opposed. Four of them he met in Greece itself, mostly in Athens, and the Pythagoreans he met largely on his journeys to southern Italy and Sicily. There is no good evidence that Persian, Indian or other “Eastern” philosophies were among them; that claim is examined in Plato’s travel and exposure to different philosophies.

Socrates: the method and the moral question

Socrates was the decisive influence. Plato knew him personally (he names himself as present at the trial, Apology 34a), and the Seventh Letter (324d–326b) calls Socrates the justest man of his time and presents his execution as one of the events that turned Plato from politics towards philosophy. From Socrates came the habit of cross-examining anyone who claimed to know what virtue, justice or courage are, and the conviction that these questions matter more than any others. See how Socrates influenced Plato and the Socratic method in the dialogues.

Heraclitus: a world in flux

Aristotle says (Metaphysics 987a29–b14) that Plato was familiar from youth with Cratylus and with the Heraclitean view that all perceptible things are always changing, so that there can be no knowledge of them. Plato took this seriously and used it in the opposite direction: if the objects of sense are always in flux, then knowledge must concern something stable. That reasoning is part of the background to the Forms. Heraclitus died long before Plato was born, so the contact came through Cratylus and the Heraclitean tradition, not from Heraclitus himself. See Heraclitus and Plato’s theory of flux and stability.

Parmenides and the Eleatics: being that does not change

Parmenides of Elea argued that what is cannot come to be or pass away. Plato’s reaction is staged in the Parmenides, where the old Parmenides meets a very young Socrates and raises serious objections to the theory of Forms (130a–135c). The most notable point is that these objections are Plato’s own: he writes the strongest criticisms of his own theory into the dialogue. See the Parmenides.

The Pythagoreans: number, soul and the mathematical sciences

In southern Italy Plato became friendly with Archytas of Tarentum, a Pythagorean mathematician and statesman (Seventh Letter 338c–d). Diogenes Laertius (III.6) adds that he sought out the Pythagoreans Philolaus and Eurytus. Aristotle says that Plato followed the Pythagoreans in many respects, such as the importance of number, while adding differences of his own. Pythagorean influence is usually seen in Plato’s emphasis on mathematics as training for philosophers, in the soul’s immortality and rebirth, and in the mathematical structure of the cosmos in the Timaeus. See Pythagorean mathematics and Plato.

The Sophists: rhetoric, relativism and Plato’s reply

The Sophists were professional teachers of rhetoric and practical skill who charged fees and travelled from city to city. Plato attacks them in the Gorgias and Protagoras, and treats their influence as a threat to the search for truth. The sharpest philosophical confrontation is with Protagoras’s claim that “man is the measure of all things”, which Socrates introduces in the Theaetetus (152a) and refutes (161c-171d) as a basis for knowledge.

Plato’s portrait should not be taken as a neutral account. He wrote as an opponent, and the Sophists were a varied group: some, like Protagoras, taught general theories; others mainly taught persuasive speaking. Our essay on who the Sophists were gives a fuller picture.

What Plato built from them: recollection, Forms and the cave

Out of these influences Plato developed his own views on knowledge. In the Meno (81a–86c) and Phaedo (72e–77a) he argues that learning is recollection: the soul knew the Forms before birth and recovers that knowledge when prompted by questions and experiences. This is sometimes called a theory of innate knowledge, though Plato’s term is anamnesis, recollection. In the Republic (514a–521b) the allegory of the cave pictures the journey from the shadows of appearance up to the knowledge of the Forms, culminating in the Form of the Good. See Plato’s reality and how to interpret the allegory.

Did Aristotle influence Plato’s later work?

Not demonstrably. Aristotle came to the Academy as a young man, around 367 BC, and stayed for about twenty years. Plato’s later dialogues were written in those years, so Aristotle was around when they were composed, but no ancient source says he shaped them. The idea that the Parmenides weaves in Aristotle’s criticisms of the Forms is unlikely: the dialogue is usually dated to about the time Aristotle arrived at the Academy aged seventeen, and the objections are best read as Plato’s own, and Aristotle’s critique of the Forms is a separate body of work in his own writings. It is better to treat the two as teacher and pupil who came to disagree. See Aristotle’s central critique of Plato.