While Thales held that water is the basic substance of all matter, and Anaximander assumed unlimited essential substances Anaximenes maintained that the basic substance in the world is air. ' The soul is air, fire is rarified air, when condensed air becomes water and finally earth and stone '.

His suggestion of condensation was highly influential in later scientific thought, but his cosmology (flat earth supported by air) was less advanced than Anaximander's who maintained that the earth floats freely in space.