Nothing, no-thing, or no thing is the complete absence of anything, as the opposite of something and an antithesis of everything. The concept of nothing has been a matter of philosophical debate since at least the 5th century BCE. 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 Isaac Newton. Nevertheless, some philosophers, like René Descartes, continued to argue against the existence of empty space until the scientific discovery of a physical vacuum.

Existentialists like Jean-Paul Sartre and Martin Heidegger (as interpreted by Sartre) have associated nothing with consciousness. Some writers^[who?] have made connections between Heidegger's concept of nothing and the nirvana of Eastern religions.

Modern science does not equate vacuum with nothing. 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.

Some would consider the study of "nothing" to be absurd. A typical response of this type is voiced by the Venetian Giacomo Casanova in conversation with his landlord, one Dr. Gozzi, who also happens to be a priest:

As everything, for him, was an article of faith, nothing, to his mind, was difficult to understand: the Great Flood had covered the entire world; before, men had the misfortune of living a thousand years; God conversed with them; Noah had taken one hundred years to build the ark; while the earth, suspended in air, stood firmly at the center of the universe that God had created out of nothingness. When I said to him, and proved to him, that the existence of nothingness was absurd, he cut me short, calling me silly.

"Nothingness" has been treated as a serious subject for a very long time. In philosophy, to avoid linguistic traps over the meaning of "nothing", a phrase such as not-being is often employed to make clear what is being discussed.

One of the earliest Western philosophers to consider nothing as a concept was Parmenides (5th century BCE), a Greek philosopher of the monist school. He argued that "nothing" cannot exist by the following line of reasoning: To speak of a thing, one has to speak of a thing that exists. If one can speak of a thing in the past, this thing must still exist (in some sense) now, and from this he concluded that there is no such thing as change. As a corollary, there can be no such things as coming-into-being, passing-out-of-being, or not-being.

Other philosophers, for instance, Socrates and Plato largely agreed with Parmenides's reasoning on nothing. Aristotle differs with Parmenides's conception of nothing and says, "Although these opinions seem to follow logically in a dialectical discussion, yet to believe them seems next door to madness when one considers the facts."