Early Greek philosopher Anixamander (ca. 610 – 545 BC) was a monist. That means he believed that ultimately there is just one sort of substance underlying all the different things we see in the physical universe. He put forth the idea that this single underlying substance of all things is something beyond our experience. He called it the ἄπειρον (<em>apeiron</em>), which means "the boundless" or "the limitless." Anaximander was reacting to the views of Thales, a previous thinker from his same town, Miletus, who had suggested that there was one underlying substance to all things, and that <u>water</u> was that essential element. Anaximander objected to Thales' thought, because water is something we all see and experience readily in the perceived world. He believed any underlying or base-level substance, from which water and any other physical stuff originated, had to be something beyond the boundaries of our present experience, or "the boundless."
One evaluation of Anaximander's views came from another Milesian philosopher who followed him: Anaximenes. Anaximenes saw the theory of Anaximander as dodging the question, "What is the main ingredient of all things in the universe." By saying, "It's boundless; it's something we don't know," had he really answered anything? So Anaximenes dismissed the view of Anaximander ... but didn't agree with Thales either. Anaximenes proposed that air was the underlying element of all physical phenomena.
You'll have to decide for yourself what you think of Anaximander's "boundless" theory.
