Material monism is a Presocratic belief which provides an explanation of the physical world by saying that all of the world's objects are composed of a single element. Among the material monists were the three Milesian philosophers: Thales, who believed that everything was composed of water; Anaximander, who believed it was apeiron; and Anaximenes, who believed it was air. For the first and last of these thinkers, however, that element was infused by mind, so it would be a mistake to call them material monists.
Although their theories were primitive, these philosophers were the first to give an explanation of the physical world without referencing the supernatural; this opened the way for much of modern science (and philosophy), which has the same goal of explaining the world without dependence on the supernatural.