Anaximander was an ancient Greek philosopher born in Miletus in 610 BC. He is best known for replacing his teacher Thales‘s belief that everything is made up of water with his own concept of an eternal, unlimited entity known as the apeiron. He is also known for creating the first sphere (a celestial globe or map of the heavens), predicted an earthquake, being the first Greek map maker, and being the author of the first surviving lines of Western philosophy.

Anaximander was the first Greek speculative astronomer which means he made guesses about things he couldn’t see based on what he could. For example, he realized that the planets make full circles and in doing so they pass beneath the earth. He also believed that the earth stayed still and sat at the center of the universe. While we now know this is not true, this theory was maintained until 1543 when Copernicus proved otherwise. He also was the first Greek philosopher to realize that the planets and stars are not all at the same distance from earth (what was known as the “celestial vault.”) As such, he was the first to conceptualize the depths of space.

He used his knowledge of Babylonian astronomy to introduce to Greece the gnomen (a raised sundial that allows you to tell the time of day as well as the distance from the solstices and equinoxes). He famously put one up in Sparta. He also successfully predicted an earthquake, and was able to warn the Spartans in time for them to leave the city and instead sleep in the fields.

Anaximander’s teacher Thales came up with the notion that there is one fundamental principle (arche) that explains how the world came into being and how it operates. While Anaximander also saw the world as having one fundamental principle, he didn’t think it was an element such as water. How could things that are not water come from water? Instead, Anaximander proposed an indefinite principle without any limits that he called the apeiron. Eternal and always in motion, he considered the apeiron divine and therefore capable of creating the world and all living things. In this way, he broke with the Greek mythological tradition of relying on the Olympic Gods to explain the world.

Anaximander needed to explain how his apeiron could produce all the diverse things of the world. He explains that while the apeiron does not have any elemental properties itself, there is a “separating off” process which allows something new to be created that does have properties. The first separation was that of a sphere of flame and dark mist and with it the opposites hot and cold. From this, the rest of the world including the earth, stars and planets also came into being. In this way, Anaximander provided a logical explanation for how a diverse world can have come into being from an initial principle.

Anaximander intriguingly describes the interplay of opposites as involving the one committing an act of injustice on the other. For example, he believed that when winter came, the cold was committing an act of injustice against the summer heat for which it would need to pay reparations. This ensured that winter would again become summer, and the world moved forward in an endless cyclical cycle.

In addition to being one of the earliest natural philosophers (i.e. scientists), he is also seen as the “Father of Cosmology” (the study of the cosmos). Through reasoning and experience, he created the first mechanical model of the universe. He understood the world as being symmetrical, assuming the earth and the heavenly bodies to be the same size. This led him to realize that the since the sun must be really far away if it looks so small to us. He believed the planets and stars were made of fire, and that the planets moved in an elliptical motion due to wind.

He saw the weather as exhibiting the same patterns as those found in the heavens. Clouds were formed when fine vapor heated leaving behind the thicker contents. Meanwhile thunder and lightning no longer came from Zeus, but rather were understood as the clashing of elements. Anaximander not only connected heaven and earth, the past and the present, with his understanding of the apieron, his process of “separating off,” and his understanding of the interplay of opposites such as hot and cold, he also used this understanding to make predictions about the future. Living in Miletus, he noticed the harbor was building up silt. From this, he predicted that just as the world had been flooded, it was currently drying up.

Anaximander believed the earth didn’t move and sat at the center of the universe. He was critical of Thales belief that the earth rests on water. If that was true, what does the water rest on? Instead, Anaximander daringly claimed the earth was unsupported, being in “equal relation to the extremes.” This seems to be the first instance of what Leibniz will later call the Principle of Sufficient Reason which states that “no fact can be real or existent… unless it has a sufficient reason why it should be thus and not otherwise.” Additionally, this belief contains the germ of the concept of gravity, which would have to wait until Newton to be recognized. Due to it’s positioning and his observations of the horizons, Anaximander concluded the earth was therefore flat and circular on top, and therefore was the shape of a cylinder.

Anaximander accounts for human life since his original substance, the apeiron, is divine and therefore infuses the world with vitality. In the “separating off” process, some things received greater concentrations of this life-force than others such as humans and animals. He also explains how it is possible for human beings to have survived initially if the first generation was born as babies. He explains that this first generation of humans was raised by other animals. He also believed all animals came from the ocean, and as such, humans came from fish. Although not an evolutionary theory, it is “proto-evolutionary” in that he is trying to account for the diversity of life forms on earth philosophically.

Anaximander was fortunate to live in Miletus where he engaged in critical discussions about the nature of things with his teacher Thales as well as his student Anaximenes. His interest in the origins of the world (cosmogony) is well rooted in ancient Greek thought. Anaximenes, his student, will go on to put forth his own system of natural philosophy in which he substitutes air for the apeiron as the fundamental principle of existence.