Classical Western Philosophy I: The Pre-Socratics, the Sophists, Socrates, Plato and Aristotle

The Sophists were fifth-century teachers who charged fees to train young Athenians in rhetoric and civic excellence (aretē). They turned philosophy from nature to human affairs and questioned whether law and morality are natural (physis) or merely conventional (nomos). Protagoras held that "man is the measure of all things, of things that are that they are, and of things that are not that they are not", which Plato's Theaetetus reads as relativism about perception and truth. Gorgias argued that nothing exists; that if anything existed it could not be known; and that if it could be known it could not be communicated.

• Method: Socrates wrote nothing and is known through Plato, Xenophon and Aristotle. His method, the elenchus, questions a person's definition of a virtue until it is shown to be inconsistent with his other beliefs; it is joined to irony (professing ignorance) and to what he called midwifery (maieutic), helping others give birth to ideas.
• Doctrines: virtue is knowledge, and no one does wrong knowingly (so wrongdoing is ignorance); the unexamined life is not worth living (Apology); care for the soul matters more than wealth or reputation. Aristotle credits Socrates with two things: inductive arguments and universal definitions.

Plato's theory of knowledge starts from the Theaetetus, which tests and rejects three definitions of knowledge: perception, true belief, and true belief with an account (logos). The Meno distinguishes true opinion from knowledge by stability: true opinions are like the statues of Daedalus that run away unless "tied down by reasoning about the cause"; the same dialogue argues from the slave boy's geometry that learning is recollection (anamnesis) of what the soul knew before birth. The Republic (Book V) grounds the distinction in objects: knowledge (epistēmē) is of what fully is, the Forms, and is infallible; opinion (doxa) is of what both is and is not, the changing sensible things.

The theory of Ideas (Forms) holds that for every general term there is an eternal, unchanging, perfect and intelligible Form, such as Beauty itself or Justice itself, of which sensible particulars are imperfect copies. Particulars are what they are by participating in (methexis) or imitating (mimēsis) their Forms. The Form of the Good is the highest: like the sun, which makes things visible and makes them grow, it makes the Forms knowable and gives them their being (the sun analogy and the allegory of the cave, Republic VI–VII). Dialectic is the method that rises by question and answer from hypotheses to an unhypothetical first principle; in the late dialogues (Phaedrus, Sophist) it becomes the method of collection and division.

The soul, in the Republic, has three parts: reason (logistikon), spirit (thumoeides) and appetite (epithumētikon), pictured in the Phaedrus as a charioteer with a noble and an unruly horse. The Phaedo argues for its immortality from the cycle of opposites, from recollection, from its affinity with the unchanging Forms, and from its essential connection with life. God, in the Timaeus, is the Demiurge, a good craftsman who shapes a pre-existing receptacle into an ordered cosmos by looking to the Forms; he is not a creator from nothing.

Aristotle classifies the sciences by their end. The theoretical sciences aim at knowledge for its own sake: first philosophy (metaphysics or theology), mathematics and physics. The practical sciences aim at action: ethics and politics. The productive sciences aim at making something: poetics, rhetoric and the crafts. Logic is not in this list at all. It is the organon, the instrument that every science uses, which is why the later collection of his logical works (Categories, On Interpretation, Prior and Posterior Analytics, Topics, Sophistical Refutations) is called the Organon.

• Critique of the Ideas (Metaphysics A.9 and elsewhere): separate Forms merely duplicate the world they are meant to explain; they cannot explain change or motion, being changeless; "participation" and "pattern" are empty words and poetic metaphors; and if the Form of Man and individual men share a likeness, a further Form is needed, and so on without end (the "third man" argument). Form is real, Aristotle concludes, but in things, not apart from them.
• Four causes (Physics II, Metaphysics A): the material cause (that out of which: the bronze of a statue), the formal cause (the form or essence: the shape of the statue), the efficient cause (the source of change: the sculptor) and the final cause (that for the sake of which: honouring a god). In natural things the formal, efficient and final causes often coincide.
• Form and matter (hylomorphism): every changeable substance is a compound of matter (hulē), which can become many things, and form (morphē or eidos), which makes it this kind of thing. Potentiality (dunamis) and actuality (energeia, entelecheia): change is the actualisation of a potential, as the acorn becomes the oak; actuality is prior to potentiality in definition, in time for the species, and in substance. This answers Parmenides: coming to be is not from sheer non-being but from potential being.
• Soul (De Anima II.1): "the first actuality of a natural body that has life potentially", the form of the living body, not a separate substance; it has nutritive (plants), sensitive (animals) and rational (humans) capacities. The active intellect (De Anima III.5) alone is said to be separable and immortal.
• God (Metaphysics Λ): the unmoved mover, pure actuality with no potentiality and no matter, whose activity is thought thinking itself (noēsis noēseōs). It moves the heavens not by pushing but as a final cause, "as the object of love moves".