Anaximander (c. 610–545 B.C.E.) was interested in the idea of what was hot and dry; this was supposed by him to be opposed to Thales’ idea of water, which was cold and wet. He reasoned that water could not be the primary substance out of which everything else was made because the primary substance must be the cause of all the others. Since water is wet and often cold, it cannot be the source of anything that is hot and dry. Therefore, Anaximander reasoned, the primary substance must be something different from both water and things that are hot and dry.

Anaximander called his primary substance, which cannot be perceived—only things that are cold and wet or hot and dry can be perceived—apeiron, or that which is eternal and causes other things to change, but does not change itself. Apeiron, in other words, is that thing which can’t be perceived itself but which is the origin of all things hot and cold, wet and dry, and for how these things change—it is responsible for everything in the world as we can and do perceive it.

According to Anaximander, we see the Sun, Moon, and stars through holes in a cold, wet vapor that encloses Earth. On Earth, wet and dry have formed land and sea, and living things are the result of the Sun’s effect on moisture. All life started in the sea, according to Anaximander, a theory that actually anticipates the theory of evolution.