The argument Anaximander used to challenge his teacher's theory of the Earth floating on water involved an idea that crops up in several strands of philosophy. If the world is supported by a body of water, then what supports the water? And then, what supports that? And so on, ad infinitum. The same pattern can be seen in arguments involving cause and effect: if something causes something else, then what caused that? This apparently unending chain is called an infinite regress.
Some philosophers saw the existence of infinite regress as proof that the universe is eternal, but many were uncomfortable with the idea and proposed that there must be an original or first cause for everything (an idea that chimes with the modern theory of the Big Bang).
For some, the first cause or 'prime mover' was an abstract idea akin to pure thought or reason, but, for medieval Christian philosophers especially, it was God: indeed, the idea of a first cause was at the heart of Thomas Aquinas's cosmological argument for the existence of God (see Existence of God: the cosmological argument).#5985•
In contrast to the school of philosophy founded by Thales at Miletus, just along the Ionian coast in the city of Ephesus lived a solitary thinker, Heraclitus, with very different philosophical views. Rather than suggesting a single element from which everything was derived, he suggested an underlying principle – that of change.
Heraclitus saw everything as consisting of opposing properties or tendencies, which come together to make up the substance of the world.
The analogy he gave was that the path up a mountain is the same as the path down.
In this theory, known as the 'unity of opposites', the tension and contradiction of opposing forces is what creates reality, but it is inherently unstable. Therefore, everything is constantly changing: everything is in flux. Just as the water in a river is constantly flowing onwards, but the river itself remains the same, that which we consider to be permanent, unchanging reality consists not of objects, but processes.#5988•
The founder of what became the 'Eleatic school' was Parmenides, who produced a counter-argument to Heraclitus's idea that everything is in flux (see Heraclitus: everything is in flux). Parmenides's argument was based on the idea that that you can't say of 'nothing' that it exists, and therefore there can never have been nothing: it cannot be true that everything came from nothing, so everything must always have existed – and will always exist since it can't become nothing either.
The universe, then, is completely full of something, which Parmenides believed was a single entity: all is one, uniform, unchanging and eternal.
This view is known as monism.#5986•