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4. The first concern of philosophy, metaphysics (What is there?)
It was about the world. How many different kinds of "stuff" make up the cosmos? Might everything be reducible to one kind of things? Can we know what it is?
The subject of metaphysics, as it would be developed in the 4th century B.C by Aristotle, had already been founded by Greek philosophers. Metaphysics refers to two distinguishable but interconnected sets of questions, first, the questions of just what there is - what really exists, and second, the question of how we know such things and whether the way we go about knowing is defensible or hopelessly defective. The first of these question is the subject of ontology; the second, epistemology. Both sets of questions preoccupied the philosophers known as the pre-Socratics.
Democritus of Abdera gave an materialist answer. It's an essentially atomic theory of reality, according to which the ultimate constituents of the universe are invisible small particles.
Thales and Heraclitus also suggested many answers of it.
The thought of the pre-Socratics was chiefly cosmological, an attempt to discern what really is. But the problem of how we can know the truth of such things(Is knowledge relative? Does it come from the gods? Why does it differ from people to people?) raises issues that the moral questioning of Socrates in the dialogues of Plato will have to address.