Monism

Monism is the metaphysical and/or religious belief that the entire universe is composed of a single "substance" and that everything we see and experience is a manifestation of one or more attributes of that substance. Because this is purely a philosophical preference, there is no empirical evidence that this is, indeed, the case. But it has an attractive simplicity that appeals to many people, and it seems to coincide with a particular religious feeling, apparently experienced by many mystics, that is sometimes described as a transcendental feeling of unity, removal of all separation between the "self" and the rest of the universe, elimination of distinctions, if not the outright dissolution of the "self". This psychological experience of oneness is compatible with a monist philosophy. However, a distinction that is completely lost on monists (such as followers of the Advaita belief system) is that there is a difference between a personal subjective experience and what is going on outside of that experience in the world. Any argument that introduces distinctions, differences, gradations, or separations has already committed philosophical suicide when debating some monists.

There are many different instances of monism that have occurred in modern history. Advaita Hinduism is Monist. On the other hand, Metaphysical materialism is also monist (but in an utterly non-spiritual way). Western philosophies have espoused monism as well from the ancient Greeks (Thales and Anaxamander) to modern times (neutral monism as described by William James and Bertrand Russell).

Not all forms of monism are religious or attempts to get at the essence of God, but they frequently find themselves nosing around that question. It only stands to reason - if there is a god, and if there is also only one thing, then god must be intrinsically tied up with that one thing.

The mystical monistic systems (e.g., Advaita, A Course in Miracles) assert that the world we believe we live in is an illusion, that all "selves" are illusions, and that the distinctions and sense of individuality and separateness that we perceive do not really exist.