"The great basic question of all philosophy, especially of more recent philosophy, is that concerning the relation of thinking and being.... the question of the relation of thinking to being, the relation of the spirit to nature - the paramount question of the whole of philosophy ... The answers which the philosophers gave to this question split them into two great camps. Those who asserted the primacy of spirit to nature... comprised the camp of idealism. The others, who regarded nature as primary, belong to the various schools of materialism." (Frederich Engels, "Ludwig Feuerbach and the End of Classical German Philosophy", 1886)
1.0 Metaphysics and Monism
1.1 The term 'metaphysics' originates from an editor of Aristotle's works to refer to the chapters that came after his studies on physics. Aristotle called such material however "first causes (aitia) and the principles (archai) of things”", representing the foundations of reality, both ontological and epistemological. It includes cosmology, the status of abstract entities (real, conceptual, nominal), debates on the nature of the mind, free will and determinism, and logic. Indeed, arguably metaphysics *is* philosophy. However, as will be illustrated, much of metaphysics is unverifiable and therefore shares the same intellectual realm as theology.
"Metaphysics is that part of philosophy which has the greatest pretensions and is exposed to the greatest suspicions. Having the avowed aim of arriving at profound truths about everything, it is sometimes held to result only in obscure nonsense about nothing." (The Concise Encyclopedia of Western Philosophy, Hutchinson, 1960)
1.2 In metaphysics idealism and physicalism represent types of monism, which various forms of pluralism can be counterposed, the most common being dualism. Note that a philosophical doctrine can be metaphysically unitary whilist being ontologically or epistemologically pluralist (e.g., Descartes famously separated the mind and body into an ontological dualism as thought and extension respectively, but the metaphysical God had both simultaneously). There is also neutral monism (e.g., Spinoza) that states that only one substance exists, but that this substance is neither physical or mental.
2.0 The Physicalist Argument
2.1 Physicalism is the metaphysical claim that everything is physical, or everything supervenes on, or is necessitated by, the physical. In the past, physicalism has been associated with materialism, however it should be clear that some physical forces are not matter (e.g., gravity). The term physicalism is broader and has a direct association with the physical sciences and many contemporary physicalists believe that the physical sciences can cover all possible knowledge. The history of physicalism includes the Carvaka school in Hinduism, the atomist and Epicurean Hellenic philosophers, most Enlightenment empiricists, nearly all atheists, Marxists, positivists, and the contemporary physicalists.
2.2 Early physicalist arguments relied on a foundational ontology; the earliest philosophers argued that everything was derived from some substance. For Thales it was water, because it included motion and change, Anaximenes suggested air, and Anaximander suggested an eternal super-element, apeiron. Atomist philosophers (e.g., Democritus, Epicurus, Lucretius) proposed a complex mechanical world where all is determined, a position taken up by a number of materialists from the French Enlightenment and Marxism. Contemporary physicalists, deriving initially from the logical positivists and logical empiricists with their demands for verification in philosophy, typically allow for a more probabilstic approach to account for quantum indeterminacy, uncertainty ("bounded rationality"), and incompleteness (Godel).
2.3 The appeal to completeness ("everything is physical") commonly relies on a hypothesis of supervenience, or often "emergentism", to include things which are typically not considered physical (e.g., ideas). The argument of supervenience is that a property A arises from ("supervenes") from other property or properties (e.g., property B, C, D) but may not be reducible to them and may have multiple realisability. Challenges against supervenience has been raised on the issue of possible worlds and changes (e.g., the lone ammonium molecule problem); perhas a more significant one can be raised concerning issues reducibility and elaboration.
"Although the position I describe denies there are psychophysical laws, it is consistent with the view that mental characteristics are in some sense dependent, or supervenient, on physical characteristics."
(Donald Davidson, Mental Events, 1970).
3.0 The Idealist Argument
3.1 Idealism is the metaphysical (or even metaideal!) claim that everything is an idea, or the result of an idea, whether this is of a subjective or objective perception, or some combination thereof. Idealism is prevalent in most Vedic and Dharmic religion, Platonic and Stoic Hellenic philosophy, medieval scholastics, most Enlightment rationalists, and nearly all theists. There are various forms of modern idealism (subjective, objective, absolute, etc).
3.2 Early idealists (Hindu, Neoplatonic) tended towards a panentheistic interpretations of idealism, arguing that there is an ultimate divine force that is within every part of nature and beyond it. For example, the Neoplatonic view, such as Plotinus, hypothesised a division between the invisible world and the phenomenal world, the former containing the transcendent absolute which emanated an eternal, perfect, essence (nous), which produces the world-soul.
3.3 In the modern form, Pluralistic idealism (Leibniz) argues that many individual minds make possible the universe; these perceptual "monads" are the true atoms of the universe. Subjective idealism combines phenomenolalist empiricism and perceptual immaterialism (Berkeley, 1710). Transcendental idealism (Kant, Schopenhauer, Strawson) argues that space and time, are not real things-in-themselves or empirical, rather they are the ordering principles of intuition by which phenomenom must be perceived (synthetic a priori); the "noumenal", thing-in-itself is beyond this. In contrast, absolute idealism (Hegel, argues that for consciousness to know the world there must be a unity of thought and being which evolves, dialectically, in stages of thought.
"Esse is percipi" ("To be is to be perceived", George Berkeley, A Treatise Concerning the Principles of Human Knowledge, 1710).
3.4 Contemporary idealists question physicalist claims concerning foundational issues such as space, time, causality, along those which that could be considered supervened (e.g., meaning and understanding) and pragmatically incommensurable (e.g., morals and aesthetics). Materialistic physicalism confronts a significant problem when it argues for completeness (Carl Hempel, 1970), especially with recent revelations in comsology; it is difficult argue simultaneously for methodological naturalism and epistemic optimism. Issues of consciousness and qualia remain a significant sticking point (c.f., "Mary The Swampy Zombie is in Your Chinese Room" discussion, Chalmers (1996), Kripke (1982)). These criticisms may be acceptable to type or token physicalism, but also to speculative notions of panphysicism, even of a teological and evolutionary variety (Nagel, 2013).
"... the Universe begins to look more like a great thought than like a great machine."
Sir James Jeans, British physicist, astronomer and mathematician (The Mysterious Universe, 1937 edition).
4.0 Never Mind, It Doesn't Matter : Non-Metaphysical Alternatives
4.1 On a metaphysical level, neither physicalism or idealism are verifiable. A universal pragmatic approach (Apel, Habermas, Lafayette*) would look at what is verifiable, with unverifiable metaphysics including physicalist, symbolist, and idealist "theologies". Within the realm of the verifiable is logical and empirical "philosophy", along the classic Socratic orientations of truth, justice, and beauty and along Popperian objective, intersubective, and subjective world-relations, validated by external correspondence, participant consensus, and sincerity of expressions respectively.
* I am including myself in this area with what I believe are unique contributions in extending historical logical empiricism to universal pragmatics, extending universal pragramtics to differentiating between theology and philosophy, and introducing a new form of speculative metaphysics in the form of symbolicism.
4.2 A related area of further investigation is the debate between empirical and logical approaches. The former argues that knowledge comes from experience or technological extensions thereof. In comparison, logical (or rationalism in this context) regards reason as the chief source of knowledge. Empiricism is inductive and sensory, rationality deductive and intellectual. Empiricists argue that rationality is an abstraction from empirical reality, rationalists argue a logical structure underlies reality which empiricism can only give an imperfect representation thereof. As this is still a matter of debate in philosophy, mathematical methods are often considered quasi-empirical.
Some Further Reading
D. Chalmers, The Conscious Mind, Oxford University Press, 1996
D. Davidson, "Mental Events", in Essays on Actions and Events, Oxford University Press, 1970, p207-223.
T. Nagel, Mind and Cosmos, Oxford University Press, 2012
J. Kim, Physicalism, Or Something Near Enough, Princeton University Press, 2005
S. Kripke, Wittgenstein on Rules and Private Language: An Elementary Exposition, Basil Blackwell, 1982
W. Quine, Two Dogmas of Empiricism, The Philosophical Review 60, 1951