The first Milesain monist was Thales. He was able to predict that there was going to be an eclipse of the sun at an exact hour and day. When that came true the colonel gave Thales all their attention. He was able to us mathematical equations to calculate when the next alignment would be. His theory was that all events have rational causes and we can understand them, helping us anticipate them. This was a crazy way to think of things because before people just thought at anytime the gods would change their world completely. They now believe that things happen in a predicable manner.
All Greeks believed that there was a substance that is common in everything in the world. This substance is called the arche, which means first or beginning. Water seems like a reasonable arche, humans can only go a few days without water and plant need water in order to grow and bloom. It makes sense why Thales thought the arche would be water. Christians even uses water for baptism, immerse a person in it to indicate a transformation from one form to another. As many philosophers do they take on students and the students that Thales took on did go along with Thales ideas about the arche. Anaximander believed the arche was boundless but then Anaximenes believed that it is air.
Anaximander's theory on the matter was that water was to common to be the arche. He claimed that if water was the source of everything then where did dust come from. Maybe the arche had no characteristics of its own and could become many different things like dust. For his theory to work it would have to be something infinite and