Acosmism, in contrast to pantheism, denies the reality of the universe, seeing it as ultimately illusory, (the prefix "a-" in Greek meaning negation; like "un-" in English), and only the infinite unmanifest Absolute as real.

This philosophy begins with the premise that there is only one real thing, and it is infinite, and non-dual; The Absolute. But the phenomenal reality of which we are normally aware is just the opposite: finite, and dualistic. And since the Absolute is the only reality, that means that everything that is not-Absolute cannot be real. Thus, according to this viewpoint, the phenomenal dualistic world is ultimately an illusion (maya to use the technical Indian term), irrespective of the apparent reality it possesses at the mundane or empirical level.

Acosmic monistic spiritual practice emphasizes attaining the Absolute through a kind of intellectual or conceptual realization. This may involve holding the thought that "I am that" (the Absolute), as in the philosophy of the Advaita Vedanta school and its recent advocates; or alternatively through a standing back and simply watching the thoughts and sensations arise and pass away; realizing all the time that they are not a part of one's true Self. Both these approaches are termed the path of jnana ("knowledge").