In philosophical systems, three forms of metaphysical monism can be identified: materialism, idealism, and neutral monism (in which the first principle is neither matter nor mind).
Parmenides (b. c. 515 B.C.E.) juxtaposes doxa (mere belief or nonbeing) and aletheia (truth or being). Being is one, unchangeable, and atemporal, and the experience of change and plurality is illusion. Parmenides's student, Zeno of Elea (c. 495–c. 430 B.C.E.), argued against the reality of plurality and motion. Plotinus (205–270 B.C.E.), the most famous and influential of the Neoplatonists, held that everything is an emanation from the One, flowing to lower and lower degrees of reality until matter is formed.
Another famous monist was Baruch Spinoza (1632–1677). In his Ethics (1677), Spinoza argues that there is at most and at least only one substance, God. His study of René Descartes (1596–1650) and the new science resulted in a metaphysical system where everything is a necessary manifestation (mode) of this single substance, which is conceived under the attributes of extension and thought.
Materialists deny the existence of any nonmaterial substance. Materialists are often, but not necessarily, atheists: some, such as the Stoics, or Thomas Hobbes (1588–1679), held that God is corporeal. Among the earliest forms of materialism was atomism. Leucippus (5th century B.C.E.) and Democritus (c. 460–c. 370 B.C.E.) believed in an infinite number of indivisible bodies (atoms) moving in a void (nonbeing). Their movements, aggregations, and interactions explain every aspect of experience, including mental life. Epicurus (Greece, 341–270 B.C.E.) developed an atomistic ethics, claiming that the pleasures of the mind and the deliverance from passions constitute human happiness. Lucretius's (c. 100 to 90–c. 55 to 53 B.C.E.) poem De rerum natura (On the nature of things) had the most developed exposition of ancient atomism. Other atomist-like schools include the Indian school of Vaisesika, founded presumably by Kasyapa (c. 2nd–3rd century C.E.), which posited an infinity of atoms, of which nine kinds are identified, which constitutes all reality. The Medieval Islamic group of Asharites known as the mutakallimun (8th–12th century) held that God was the direct and continuous cause of all created beings (composed of atoms) and of the maintenance of each atom, from instant to instant. This doctrine of atomic time presaged Descartes and the Occasionalists, though without the commitment to a transcendent God.
Seventeenth-and eighteenth-century materialism arose with the scientific revolution's mechanical philosophy. Most natural philosophers were not materialist monists (because they believed in a transcendent God and in incorporeal souls), yet there are exceptions. Giordano Bruno (1548–1600), who was burned as a heretic, presented a pantheistic system in which the world and its soul are one. Thomas Hobbes in Leviathan (1651) developed a geometrical account of natural, human, and political science, where he argued that reality is only bodies in motion and these explain perception, our human ideas and volitions, and the body politic.
During the Enlightenment materialists like Julien Offroy de La Mettrie (1709–1751) and Baron Paul-Henri-Dietrich d'Holbach (1723–1789) wrote about the material and "mechanical" nature of man and rejected any immaterial God. Some Enlightenment philosophers (Denis Diderot, Holbach) were atheists, others opted for deism, or the belief that God acted exclusively through natural laws. The borders between deism, pantheism, or downright atheism were often quite blurred (as in Anthony Ashley Cooper, third earl of Shaftesbury, or Jean Le Rond d'Alembert).
The first half of the nineteenth century saw a strong idealistic and romantic reaction to materialism, yet materialism returned with a vengeance in the second half with the successes of the theory of evolution by natural selection and of Marxism. Karl Marx (1818–1883) focused on economics, yet, in reacting against Hegelian idealism, endorsed a metaphysical materialism and atheism. In Marx, ideas (intellectual contents) are determined and explained through the material, economic processes of production and ownership, upon which rise social and political superstructures.
On the Origin of Species by Means of Natural Selection: Or, The Preservation of Favoured Races in the Struggle for Life (1859) by Charles Darwin (1809–1882) made it possible, in Richard Dawkins's words in The Blind Watchmaker, "to be an intellectually fulfilled atheist." Darwin was held by many to have produced a fully naturalistic and scientifically robust explanation of the nature of life. Darwinian evolution and natural selection were taken to be a serious challenge to explanations of the creation and development of life by a designing God. While many aspects of Darwin's own theory underwent major revisions with the discoveries of molecular biology and genetics, natural selection has lost nothing of its power as an explanatory tool in contemporary evolutionary biology and is used in many fields to explain structures and functions that are seemingly designed.
Idealism is metaphysical monism that rejects the existence of matter and founds the experience of matter on the mental. Ancient forms of idealism can be found in Buddhist schools.
The Yogacara (or Vijnanavada) school started around the fifth century C.E. in India. Its central doctrine is that only consciousness (vijnanamatra) is real, that thought or mind is the ultimate reality. External things do not exist; nothing exists outside the mind. Ultimately, the purified, undifferentiated state of the mind without objects or thought processes is what constitutes "Buddhahood." Among the principal figures were the brothers Asanga and Vasubandhu.
The Zen, or Chan school was founded by the Indian monk Bodhidharma, who traveled to China around 520 C.E. (according to the tradition). Zen stresses the use of meditation to experience the unity and indistinctness of reality, which cannot be understood otherwise (any form of verbalization or conceptualization falls into the trap of dualism).
In the Western tradition, idealism was reprised by George Berkeley (1685–1753), bishop of Cloyne, who held that all we can know are the ideas in our minds. All objects of perception, including matter, are only ideas produced by God. German idealism started later with Johann Gottlieb Fichte's (1762–1814) transcendental idealism, which he saw as a development of Immanuel Kant's (1724–1804) ideas. For Fichte, God is the All, and particular objects result from reflection or self-consciousness through which the infinite unity is broken up. Friedrich Wilhelm Joseph von Schelling (1775–1854) held that there is a unity between ideal and real. Absolute idealism explains the process of division of consciousness and nature, and their return to unity.
The culmination of German idealism came with Georg Wilhelm Friedrich Hegel (1770–1831), who believed that understanding Geist (Absolute Spirit) would overcome all (Kant's) contradictions in the realms of reason and science. Geist is the principle of reality that makes the universe intelligible as an eternal cyclical process whereby Geist comes to know itself, first through its own thinking, then through nature, and finally through finite spirits and their self-expression in history and their self-discovery in art, in religion, and in philosophy. Later in the century, Hegel's doctrines were developed by a group of English idealists, most notably Francis Bradley and John McTaggart Ellis.
In contemporary philosophy, monism is mostly materialism or physicalism. In psychology and philosophy of mind, it is held by many that the mental is reducible to the physical and may be explained in terms of physical laws. Eliminative materialism (Patricia and Paul Churchland) denies that the mental exists and claims that all "mental" talk will be ultimately eliminated as science progresses. Functionalism is a noneliminativist form of materialism claiming that descriptions of mental events and their intentional natures may be explained by systemic relations among the parts of the material brain (Hilary Putnam and William Lycan).
Anomalous monism (Donald Davidson and John Searle) holds that mental events are ultimately identical to brain states, but that there are no laws (hence anomalous) that connect brain states to mental states.
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Francesca di Poppa