The first philosophers were primarily interested in explaining everything in terms of one simple principle.
Thales: Water is the most fundamental element.II. Permanence vs Change
Anaximander: An infinite, unobservable, amorphus indefinite is the the most fundamental element.
Pythagoras: Numbers. Also started a religious cult.
In addition to this, we have the problem of change. Things around us appear to change, but things appear to stay the same as well. The water in a river is never the same, but the river itself persists. A person's physical body changes constantly, but the person persists over time. What accounts for this? What is fundamental? Change or permanence?
Heraclitus: Change is fundamental. "You can't step into the same river twice."III. Responses to Permanence
Parmenides: Permanence is fundamental. Existence "is", and non-existence "is not". Since change would involve something coming to be that did not exist (such as a property), and non-existence is not, then change is impossible. Everything is One.
Zeno: Follow of Parmenides. Agreed that change is impossible, and formulated his famous paradoxes to prove it.
In many ways, the next philosophers tried to answer Parmenides' declaration of the impossibility of change, because it at least seems like change occurs. They attempted to combine elements into one (thus in keeping with Parmenides argument that all is One), or claim that there are several fundamental elements.
Anaxagoras: All elements exist in a single mass. Nous (mind) separates the elements into their individual forms.
Empedocles: Fire, air, earth, and water are fundamental.
Democritus: Atoms are fundamental. Everything is composed of groups of small particles that cannot be broken down any further.