Materialism
Especially, a materialist does not believe in supernatural entities. Many of history's materialists thinkers have found the idea of a spirit either meaningless or not scientifically proven. By definition, these thinkers are therefore also atheists, though not all atheists are materialists, and there have even been some historically important Materialist thinkers who believe in a Pantheist idea of God (such as Baruch Spinoza and Friedrich Engels).