This philosophy begins with the premise that there is only one Reality that is infinite, non-dual, and blissful. But the phenomenal reality of which we are normally aware is just the opposite: finite, dualistic, and full of pain and suffering. 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 emphasises attaining the Absolute through a kind of intellectual or conceptual realisation. This may involve holding the thought that "I am that" (the Absolute), as in the 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; realising all the time that they are not a part of one's true Self. Both these approaches are termed the path of jnana ("knowledge").
Indian philosophies were and are concerned not so much with the manifest reality we see about us, but with the unmanifest Absolute Transcendent. What matters is simply the practical attainment of a state of this universal, transcendent, transpersonal existence. In that state, according to Adi Shankara, there is no difference between Self and God; there is only the Absolute (Brahman).