something like a clock. In response to Thales, he postulated as the first principle an undefined, unlimited substance without qualities (apeiron), out of which the primary opposites, hot and cold, moist and dry, became differentiated. His answer was an attempt to explain observable changes by attributing them to a single source that transforms to various elements. Like Thales, he provided a naturalistic explanation for phenomena previously given supernatural explanations. He is also known for speculating on the origin of mankind. He proclaimed that the earth is not situated in another structure but lies unsupported in the middle of the universe. Further, he developed a rudimentary evolutionary explanation for biodiversity in which constant universal powers affected the lives of animals. According to Giorgio de Santillana, a philosophy professor at the Massachusetts Institute of Technology, Anaximander's conception of a universe governed by laws shaped the philosophical thinking of centuries to come and was as important as the discovery of fire or Einstein's breakthroughs in science. Anaximenes Little is known of Anaximenes' (585–525 BCE) life. He was a younger contemporary and friend of Anaximander, and the two worked together on various intellectual projects. He also wrote a book on nature in prose. Anaximenes took for his principle aēr (air), conceiving it as being modified, via thickening and thinning, into the other classical elements: fire, wind, clouds, water, and earth. While his theory resembled that of Anaximander, as they both claimed a single source of the universe, Anaximenes suggested sophisticated mechanisms in which air is transformed to other elements, mainly because of changes of density. Since the classical era, he was considered the father of naturalistic explanations. Anaximenes expanded Anaximander's attempt to find a unitary cause explaining natural phenomena both living and nonliving, without, according to James Warren, having to "reduce living things in some way to mere locations of material change". Xenophanes Xenophanes was born in Colophon, an Ionian town near Miletus. He was a well-traveled poet whose primary interests were theology and epistemology. Concerning theology, he pointed out that we did not know whether there was one god or many gods, or in such case whether there was a hierarchy among them. To critique the anthropomorphic representation of the gods by his contemporary Greeks, he pointed out that different nations depicted their gods as looking like themselves. He famously said that if oxen, horses, or lions could draw, they would draw their