Any view claiming to find unity in a certain sphere where it might not have been expected.
The main forms of monism have been: a strong form, claiming that there is only one object (Eleaticism, Baruch de Spinoza (1632-1677), Georg Wilhelm Friedrich Hegel (1770-1831)); and a weaker form, claiming that there is only one kind of object, and in particular that matter and mind are not two independent kinds of thing (materialism, identity theory of mind, neutral monism, and subjective idealism in which matter is rejected, though minds and ideas are considered different).
Other forms of monism may say, for example, that things are related together or unified in their being governed by a simple law or principle (Heraclitus(fl.500 BC); or that there is only one proper kind of explanation of covering law model; or one basic ground for our duties (for example utilitarianism).
Also see: anomalous monism
There are two sorts of definitions for monism:
- The wide definition: a philosophy is monistic if it postulates unity of the origin of all things; all existing things return to a source that is distinct from them.
- The restricted definition: this requires not only unity of origin but also unity of substance and essence.
Although the term monism is derived from Western philosophy to typify positions in the mind–body problem, it has also been used to typify religious traditions. In modern Hinduism, the term “absolute monism” is used for Advaita Vedanta.
The term monism was introduced in the 18th century by Christian von Wolff in his work Logic (1728), to designate types of philosophical thought in which the attempt was made to eliminate the dichotomy of body and mind and explain all phenomena by one unifying principle, or as manifestations of a single substance.
The mind–body problem in philosophy examines the relationship between mind and matter, and in particular the relationship between consciousness and the brain. The problem was addressed by René Descartes in the 17th century, resulting in Cartesian dualism, and by pre-Aristotelian philosophers, in Avicennian philosophy, and in earlier Asian and more specifically Indian traditions.
It was later also applied to the theory of absolute identity set forth by Hegel and Schelling. Thereafter the term was more broadly used, for any theory postulating a unifying principle. The opponent thesis of dualism also was broadened, to include pluralism. According to Urmson, as a result of this extended use, the term is “systematically ambiguous”.
According to Jonathan Schaffer, monism lost popularity due to the emergence of analytic philosophy in the early twentieth century, which revolted against the neo-Hegelians. Carnap and Ayer, who were strong proponents of positivism, “ridiculed the whole question as incoherent mysticism”.
The mind–body problem has reemerged in social psychology and related fields, with the interest in mind–body interaction and the rejection of Cartesian mind–body dualism in the identity thesis, a modern form of monism. Monism is also still relevant to the philosophy of mind, where various positions are defended.
Different types of monism include:
- Substance monism, “the view that the apparent plurality of substances is due to different states or appearances of a single substance”
- Attributive monism, “the view that whatever the number of substances, they are of a single ultimate kind”
- Partial monism, “within a given realm of being (however many there may be) there is only one substance”
- Existence monism, “the view that there is only one concrete object token (The One, “Τὸ Ἕν” or the Monad)”
- Priority monism, “the whole is prior to its parts” or “the world has parts, but the parts are dependent fragments of an integrated whole”
- Property monism, “the view that all properties are of a single type (e.g., only physical properties exist)”
- Genus monism, “the doctrine that there is a highest category; e.g., being”
Views contrasting with monism are:
- Metaphysical dualism, which asserts that there are two ultimately irreconcilable substances or realities such as Good and Evil, for example, Manichaeism.
- Metaphysical pluralism, which asserts three or more fundamental substances or realities.
- Metaphysical nihilism, negates any of the above categories (substances, properties, concrete objects, etc.).
Monism in modern philosophy of mind can be divided into three broad categories:
- Idealist, mentalistic monism, which holds that only mind or spirit exists. 
- Neutral monism, which holds that one sort of thing fundamentally exists, to which both the mental and the physical can be reduced
- Material monism (also called Physicalism and materialism), which holds that the material world is primary, and consciousness arises through the interaction with the material world
- Eliminative Materialism, according to which everything is physical and mental things do not exist
- Reductive physicalism, according to which mental things do exist and are a kind of physical thing[note 1]
Certain positions do not fit easily into the above categories, such as functionalism, anomalous monism, and reflexive monism. Moreover, they do not define the meaning of “real”.
While the lack of information makes it difficult in some cases to be sure of the details, the following pre-Socratic philosophers thought in monistic terms:
- Thales: Water
- Anaximander: Apeiron (meaning ‘the undefined infinite’). Reality is some, one thing, but we cannot know what.
- Anaximenes of Miletus: Air
- Heraclitus: Change, symbolized by fire (in that everything is in constant flux).
- Parmenides: Being or Reality is an unmoving perfect sphere, unchanging, undivided.
- Neopythagorians such as Apollonius of Tyana centered their cosmologies on the Monad or One.
- Stoics taught that there is only one substance, identified as God.
- Middle Platonism under such works as those by Numenius taught that the Universe emanates from the Monad or One.
- Neoplatonism is monistic. Plotinus taught that there was an ineffable transcendent god, ‘The One,’ of which subsequent realities were emanations. From The One emanates the Divine Mind (Nous), the Cosmic Soul (Psyche), and the World (Cosmos).
- Giordano Bruno
- Baruch Spinoza
- Gottfried Wilhelm Leibniz
- Friedrich Wilhelm Joseph Schelling
- Johann Gottlieb Fichte
- Georg Wilhelm Friedrich Hegel
- F. H. Bradley
- Arthur Schopenhauer
- Ernst Haeckel
- Alan Watts
- Herbert Spencer
- Friedrich Engels
- Karl Marx
- Georgi Plekhanov
- Ernst Mach
- Wilhelm Ostwald
- Alexander Bogdanov
- Bertrand Russell
- B.F. Skinner
- Giacomo Leopardi
- Gilbert Ryle
- Jonathan Schaffer
- Alfred North Whitehead
- Christopher Langan