Nothing, no-thing, or no thing, is the complete absence of anything; the opposite of everything, or its complement. The concept of nothing has been a matter of philosophical debate since at least the 5th century BC. Early Greek philosophers argued that it was impossible for nothing to exist. The atomists allowed nothing but only in the spaces between the invisibly small atoms. For them, all space was filled with atoms. Aristotle took the view that there exists matter and there exists space, a receptacle into which matter objects can be placed. This became the paradigm for classical scientists of the modern age like Newton. Nevertheless, some philosophers, like Descartes, continued to argue against the existence of empty space until the scientific discovery of a physical vacuum.

Existentialists like Sartre and Heidegger (as interpreted by Sartre) have associated nothing with consciousness. Some writers have made connections between Heidegger's concept of nothing and the nirvana of Eastern religions.

Modern science does not equate vacuum with nothing. Indeed, the vacuum in quantum field theory is filled with virtual particles. The quantum vacuum is often viewed as a modern version of an aether theory.