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Metaphysics is a traditional branch of philosophy concerned with explaining the fundamental nature of being and the world that encompasses it, although the term is not easily defined. Traditionally, metaphysics attempts to answer two basic questions in the broadest possible terms:
- Ultimately, what is there?
- What is it like?
A person who studies metaphysics is called a metaphysicist or a metaphysician. The metaphysicist attempts to clarify the fundamental notions by which people understand the world, e.g., existence, objects and their properties, space and time, cause and effect, and possibility. A central branch of metaphysics is ontology, the investigation into the basic categories of being and how they relate to each other. Another central branch of metaphysics is cosmology, the study of the origin, fundamental structure, nature, and dynamics of the universe. Some include epistemology as another central focus of metaphysics, but others question this.
Prior to the modern history of science, scientific questions were addressed as a part of metaphysics known as natural philosophy. Originally, the term "science" (Latin scientia) simply meant "knowledge". The scientific method, however, transformed natural philosophy into an empirical activity deriving from experiment unlike the rest of philosophy. By the end of the 18th century, it had begun to be called "science" to distinguish it from philosophy. Thereafter, metaphysics denoted philosophical enquiry of a non-empirical character into the nature of existence. Some philosophers of science, such as the neo-positivists, say that natural science rejects the study of metaphysics, while other philosophers of science strongly disagree.
The word "metaphysics" derives from the Greek words μετά (metá, "beyond", "upon" or "after") and φυσικά (physiká, "physics"). It was first used as the title for several of Aristotle's works, because they were usually anthologized after the works on physics in complete editions. The prefix meta- ("after") indicates that these works come "after" the chapters on physics. However, Aristotle himself did not call the subject of these books "Metaphysics": he referred to it as "first philosophy." The editor of Aristotle's works, Andronicus of Rhodes, is thought to have placed the books on first philosophy right after another work, Physics, and called them τὰ μετὰ τὰ φυσικὰ βιβλία (ta meta ta physika biblia) or "the books that come after the [books on] physics". This was misread by Latin scholiasts, who thought it meant "the science of what is beyond the physical".
However, once the name was given, the commentators sought to find intrinsic reasons for its appropriateness. For instance, it was understood to mean "the science of the world beyond nature" (physis in Greek), that is, the science of the immaterial. Again, it was understood to refer to the chronological or pedagogical order among our philosophical studies, so that the "metaphysical sciences" would mean "those that we study after having mastered the sciences that deal with the physical world" (St. Thomas Aquinas, Expositio in librum Boethii De hebdomadibus, V, 1).
There is a widespread use of the term in current popular literature which replicates this understanding, i.e. that the metaphysical equates to the non-physical: thus, "metaphysical healing" means healing by means of remedies that are not physical.
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Cosmology and cosmogony
Metaphysical Cosmology is the branch of metaphysics that deals with the world as the totality of all phenomena in space and time. Historically, it has had quite a broad scope, and in many cases was founded in religion. The ancient Greeks drew no distinction between this use and their model for the cosmos. However, in modern times it addresses questions about the Universe which are beyond the scope of the physical sciences. It is distinguished from religious cosmology in that it approaches these questions using philosophical methods (e.g. dialectics).
Cosmogony deals specifically with the origin of the universe.
Modern metaphysical cosmology and cosmogony try to address questions such as:
- What is the origin of the Universe? What is its first cause? Is its existence necessary? (see monism, pantheism, emanationism and creationism)
- What are the ultimate material components of the Universe? (see mechanism, dynamism, hylomorphism, atomism)
- What is the ultimate reason for the existence of the Universe? Does the cosmos have a purpose? (see teleology)
Determinism and free will
Determinism is the philosophical proposition that every event, including human cognition, decision and action, is causally determined by an unbroken chain of prior occurrences. It holds that nothing happens that has not already been determined. The principal consequence of the deterministic claim is that it poses a challenge to the existence of free will.
The problem of free will is the problem of whether rational agents exercise control over their own actions and decisions. Addressing this problem requires understanding the relation between freedom and causation, and determining whether the laws of nature are causally deterministic. Some philosophers, known as Incompatibilists, view determinism and free will as mutually exclusive. If they believe in determinism, they will therefore believe free will to be an illusion, a position known as Hard Determinism. Proponents range from Baruch Spinoza to Ted Honderich.
Others, labeled Compatibilists (or "Soft Determinists"), believe that the two ideas can be reconciled coherently. Adherents of this view include Thomas Hobbes and many modern philosophers such as John Martin Fischer.
Incompatibilists who accept free will but reject determinism are called Libertarians, a term not to be confused with the political sense. Robert Kane and Alvin Plantinga are modern defenders of this theory.
Identity and change
The Greeks took some extreme positions on the nature of change: Parmenides denied that change occurs at all, while Heraclitus thought change was ubiquitous: "[Y]ou cannot step into the same river twice."
Identity, sometimes called Numerical Identity, is the relation that a "thing" bears to itself, and which no "thing" bears to anything other than itself (cf. sameness). According to Leibniz, if some object x is identical to some object y, then any property that x has, y will have as well. However, it seems, too, that objects can change over time. If one were to look at a tree one day, and the tree later lost a leaf, it would seem that one could still be looking at that same tree. Two rival theories to account for the relationship between change and identity are Perdurantism, which treats the tree as a series of tree-stages, and Endurantism, which maintains that the tree—the same tree—is present at every stage in its history.
Mind and matter
The nature of matter was a problem in its own right in early philosophy. Aristotle himself introduced the idea of matter in general to the Western world, adapting the term hyle, which originally meant "lumber." Early debates centered on identifying a single underlying principle. Water was claimed by Thales, air by Anaximenes, Apeiron (the Boundless) by Anaximander, fire by Heraclitus. Democritus, in conjunction with his mentor, Leucippus, conceived of an atomic theory many centuries before it was accepted by modern science. It is worth noting, however, that the grounds necessary to ensure validity to the proposed theory's veridical nature were not scientific, but just as philosophical as those traditions espoused by Thales and Anaximander.
The nature of the mind and its relation to the body has been seen as more of a problem as science has progressed in its mechanistic understanding of the brain and body. Proposed solutions often have ramifications about the nature of mind as a whole. René Descartes proposed substance dualism, a theory in which mind and body are essentially different, with the mind having some of the attributes traditionally assigned to the soul, in the seventeenth century. This creates a conceptual puzzle about how the two interact (which has received some strange answers, such as occasionalism). Evidence of a close relationship between brain and mind, such as the Phineas Gage case, have made this form of dualism increasingly unpopular.
Another proposal discussing the mind-body problem is idealism, in which the material is sweepingly eliminated in favor of the mental. Idealists, such as George Berkeley, claim that material objects do not exist unless perceived and only as perceptions. The "German idealists" such as Fichte, Hegel and Schopenhauer took Kant as their starting-point, although it is debatable how much of an idealist Kant himself was. Idealism is also a common theme in Eastern philosophy. Related ideas are panpsychism and panexperientialism, which say everything has a mind rather than everything exists in a mind. Alfred North Whitehead was a twentieth-century exponent of this approach.
Idealism is a monistic theory which holds that there is a single universal substance or principle. Neutral monism, associated in different forms with Baruch Spinoza and Bertrand Russell, seeks to be less extreme than idealism, and to avoid the problems of substance dualism. It claims that existence consists of a single substance that in itself is neither mental nor physical, but is capable of mental and physical aspects or attributes – thus it implies a dual-aspect theory.
For the last one hundred years, the dominant metaphysics has without a doubt been materialistic monism. Type identity theory, token identity theory, functionalism, reductive physicalism, nonreductive physicalism, eliminative materialism, anomalous monism, property dualism, epiphenomenalism and emergence are just some of the candidates for a scientifically informed account of the mind. (It should be noted that while many of these positions are dualisms, none of them are substance dualism.)
Prominent recent philosophers of mind include David Armstrong, Ned Block, David Chalmers, Patricia and Paul Churchland, Donald Davidson, Daniel Dennett, Fred Dretske, Douglas Hofstadter, Jerry Fodor, David Lewis, Thomas Nagel, Hilary Putnam, John Searle, John Smart, Ludwig Wittgenstein, and Fred Alan Wolf.
Necessity and possibility
Metaphysicists investigate questions about the ways the world could have been. David Lewis, in "On the Plurality of Worlds," endorsed a view called Concrete Modal realism, according to which facts about how things could have been are made true by other concrete worlds, just as in ours, in which things are different. Other philosophers, such as Gottfried Leibniz, have dealt with the idea of possible worlds as well. The idea of necessity is that any necessary fact is true across all possible worlds. A possible fact is true in some possible world, even if not in the actual world. For example, it is possible that cats could have had two tails, or that any particular apple could have not existed. By contrast, certain propositions seem necessarily true, such as analytic propositions, e.g., "All bachelors are unmarried." The particular example of analytic truth being necessary is not universally held among philosophers. A less controversial view might be that self-identity is necessary, as it seems fundamentally incoherent to claim that for any x, it is not identical to itself; this is known as the law of identity, a putative "first principle". Aristotle describes the principle of non-contradiction, "It is impossible that the same quality should both belong and not belong to the same thing ... This is the most certain of all principles ... Wherefore they who demonstrate refer to this as an ultimate opinion. For it is by nature the source of all the other axioms."
Religion and spirituality
Theology is the study of a god or gods and the nature of the divine. Whether there is a god (monotheism), many gods (polytheism) or no gods (atheism), or whether it is unknown or unknowable whether any gods exist (agnosticism; apophatic theology), and whether a divine entity directly intervenes in the world (theism), or its sole function is to be the first cause of the universe (deism); these and whether a god or gods and the world are different (as in panentheism and dualism), or are identical (as in pantheism), are some of the primary metaphysical questions concerning philosophy of religion.
Within the standard Western philosophical tradition, theology reached its peak under the medieval school of thought known as scholasticism, which focused primarily on the metaphysical aspects of Christianity. The work of the scholastics is still an integral part of modern philosophy, with key figures such as Thomas Aquinas still playing an important role in the philosophy of religion.
The nature of metaphysics
Some philosophers, such as Amie Thomasson, have argued that many metaphysical questions can be dissolved just by looking at the way we use words; others, such as Ted Sider, have argued that metaphysical questions are substantive, and that we can make progress toward answering them by comparing theories according to a range of theoretical virtues inspired by the sciences, such as simplicity and explanatory power.
History and schools of metaphysics
Pre-Socratic metaphysics in Greece
The first known philosopher, according to Aristotle, is Thales of Miletus. Rejecting mythological and divine explanations, he sought a single first cause or Arche (origin or beginning) under which all phenomena could be explained, and concluded that this first cause was in fact moisture or water. Thales also taught that the world is harmonious, has a harmonious structure, and thus is intelligible to rational understanding. Other Miletians, such as Anaximander and Anaximenes, also had a monistic conception of the first cause.
Another school was the Eleatics, Italy. The group was founded in the early fifth century BCE by Parmenides, and included Zeno of Elea and Melissus of Samos. Methodologically, the Eleatics were broadly rationalist, and took logical standards of clarity and necessity to be the criteria of truth. Parmenides' chief doctrine was that reality is a single unchanging and universal Being. Zeno used reductio ad absurdum, to demonstrate the illusory nature of change and time in his paradoxes.
Heraclitus of Ephesus, in contrast, made change central, teaching that "all things flow". His philosophy, expressed in brief aphorisms, is quite cryptic. For instance, he also taught the unity of opposites.
Socrates and Plato
His pupil, Plato is famous for his theory of forms (which he places in the mouth of Socrates in the dialogues he wrote to expound it). Platonic realism (also considered a form of idealism) is considered to be a solution to the problem of universals; i.e., what particular objects have in common is that they share a specific Form which is universal to all others of their respective kind.
The theory has a number of other aspects:
- Epistemological: knowledge of the Forms is more certain than mere sensory data.
- Ethical: The Form of the Good sets an objective standard for morality.
- Time and Change: The world of the Forms is eternal and unchanging. Time and change belong only to the lower sensory world. "Time is a moving image of Eternity".
- Abstract objects and mathematics: Numbers, geometrical figures, etc., exist mind-independently in the World of Forms.
Platonism developed into Neoplatonism, a philosophy with a monotheistic and mystical flavour that survived well into the early Christian era.
Plato's pupil Aristotle wrote widely on almost every subject, including metaphysics. His solution to the problem of universals contrasts with Plato's. Whereas Platonic Forms exist in a separate realm, and may exist uninstantiated in visible things, Aristotelian essences "indwell" in particulars.
The Aristotelian theory of change and causality stretches to four causes: the material, formal, efficient and final. The efficient cause corresponds to what is now known as a cause simpliciter. Final causes are explicitly teleological, a concept now regarded as controversial in science. The Matter/Form dichotomy was to become highly influential in later philosophy as the substance/essence distinction.
The opening arguments in Aristotle's Metaphysics, Book I, revolve around the senses, knowledge, experience, theory, and wisdom. The first main focus in the Metaphysics is attempting to determine how intellect "advances from sensation through memory, experience, and art, to theoretical knowledge". Aristotle claims that eyesight provides us with the capability to recognize and remember experiences, while sound allows us to learn.
Metaphysics in India
Sāṃkhya is an ancient system of Indian philosophy based on a dualism involving the ultimate principles of consciousness and matter. It is described as the rationalist school of Indian philosophy. It is most related to the Yoga school of Hinduism, and its method was most influential on the development of Early Buddhism.
The Sāmkhya is an enumerationist philosophy whose epistemology accepts three of six pramanas (proofs) as the only reliable means of gaining knowledge. These include pratyakṣa (perception), anumāṇa (inference) and śabda (āptavacana, word/testimony of reliable sources).
Samkhya is strongly dualist. Sāmkhya philosophy regards the universe as consisting of two realities; puruṣa (consciousness) and prakṛti (matter). Jiva (a living being) is that state in which puruṣa is bonded to prakṛti in some form. This fusion, state the Samkhya scholars, led to the emergence of buddhi ("spiritual awareness") and ahaṅkāra (ego consciousness). The universe is described by this school as one created by purusa-prakṛti entities infused with various permutations and combinations of variously enumerated elements, senses, feelings, activity and mind. During the state of imbalance, one of more constituents overwhelm the others, creating a form of bondage, particularly of the mind. The end of this imbalance, bondage is called liberation, or moksha, by the Samkhya school.
The existence of God or supreme being is not directly asserted, nor considered relevant by the Samkhya philosophers. Sāṃkhya denies the final cause of Ishvara (God). While the Samkhya school considers the Vedas as a reliable source of knowledge, it is an atheistic philosophy according to Paul Deussen and other scholars. A key difference between Samkhya and Yoga schools, state scholars, is that Yoga school accepts a "personal, yet essentially inactive, deity" or "personal god".
Samkhya is known for its theory of guṇas (qualities, innate tendencies). Guṇa, it states, are of three types: sattva being good, compassionate, illuminating, positive, and constructive; rajas is one of activity, chaotic, passion, impulsive, potentially good or bad; and tamas being the quality of darkness, ignorance, destructive, lethargic, negative. Everything, all life forms and human beings, state Samkhya scholars, have these three guṇas, but in different proportions. The interplay of these guṇas defines the character of someone or something, of nature and determines the progress of life. The Samkhya theory of guṇas was widely discussed, developed and refined by various schools of Indian philosophies, including Buddhism. Samkhya's philosophical treatises also influenced the development of various theories of Hindu ethics.
Realization of the nature of Self-identity is the principal object of the Vedanta system of Indian metaphysics. In the Upanishads, self-consciousness is not the first-person indexical self-awareness or the self-awareness which is self-reference without identification, and also not the self-consciousness which as a kind of desire is satisfied by another self-consciousness. It is Self-realisation; the realisation of the Self consisting of consciousness that leads all else.
The word Self-consciousness in the Upanishads means the knowledge about the existence and nature of Brahman. It means the consciousness of our own real being, the primary reality. Self-consciousness means Self-knowledge, the knowledge of Prajna i.e. of Prana which is Brahman. According to the Upanishads the Atman or Paramatman is phenomenally unknowable; it is the object of realisation. The Atman is unknowable in its essential nature; it is unknowable in its essential nature because it is the eternal subject who knows about everything including itself. The Atman is the knower and also the known.
Metaphysicists regard the Self either to be distinct from the Absolute or entirely identical with the Absolute. They have given form to three schools of thought – a) the Dualistic school, b) the Quasi-dualistic school and c) the Monistic school, as the result of their varying mystical experiences. Prakrti and Atman, when treated as two separate and distinct aspects form the basis of the Dualism of the Shvetashvatara Upanishad. Quasi-dualism is reflected in the Vaishnavite-monotheism of Ramanuja and the absolute Monism, in the teachings of Adi Shankara.
Self-consciousness is the Fourth state of consciousness or Turiya, the first three being Vaisvanara, Taijasa and Prajna. These are the four states of individual consciousness.
There are three distinct stages leading to Self-realisation. The First stage is in mystically apprehending the glory of the Self within us as though we were distinct from it. The Second stage is in identifying the "I-within" with the Self, that we are in essential nature entirely identical with the pure Self. The Third stage is in realising that the Atman is Brahman, that there is no difference between the Self and the Absolute. The Fourth stage is in realising "I am the Absolute" - Aham Brahman Asmi. The Fifth stage is in realising that Brahman is the "All" that exists, as also that which does not exist.
Scholasticism and the Middle Ages
Between about 1100 and 1500, philosophy as a discipline took place as part of the Catholic church's teaching system, known as scholasticism. Scholastic philosophy took place within an established framework blending Christian theology with Aristotelian teachings. Although fundamental orthodoxies could not be challenged, there were nonetheless deep metaphysical disagreements, particularly over the problem of universals, which engaged Duns Scotus and Pierre Abelard. William of Ockham is remembered for his principle of ontological parsimony.
Rationalism and Continental Rationalism
In the early modern period (17th and 18th centuries), the system-building scope of philosophy is often linked to the rationalist method of philosophy, that is the technique of deducing the nature of the world by pure reason. The scholastic concepts of substance and accident were employed.
- Leibniz proposed in his Monadology a plurality of non-interacting substances.
- Descartes is famous for his Dualism of material and mental substances.
- Spinoza believed reality was a single substance of God-or-nature.
British empiricism marked something of a reaction to rationalist and system-building philosophy, or speculative metaphysics as it was pejoratively termed. The sceptic David Hume famously declared that most metaphysics should be consigned to the flames (see below). Hume was notorious among his contemporaries as one of the first philosophers to openly doubt religion, but is better known now for his critique of causality. John Stuart Mill, Thomas Reid and John Locke were less sceptical, embracing a more cautious style of metaphysics based on realism, common sense and science. Other philosophers, notably George Berkeley were led from empiricism to idealistic metaphysics.
Immanuel Kant attempted a grand synthesis and revision of the trends already mentioned: scholastic philosophy, systematic metaphysics, and skeptical empiricism, not to forget the burgeoning science of his day. As did the systems builders, he had an overarching framework in which all questions were to be addressed. Like Hume, who famously woke him from his 'dogmatic slumbers', he was suspicious of metaphysical speculation, and also places much emphasis on the limitations of the human mind.
Kant saw rationalist philosophers as aiming for a kind of metaphysical knowledge he defined as the synthetic apriori—that is knowledge that does not come from the senses (it is a priori) but is nonetheless about reality (synthetic). Inasmuch as it is about reality, it differs from abstract mathematical propositions (which he terms analytical apriori), and being apriori it is distinct from empirical, scientific knowledge (which he terms synthetic aposteriori). The only synthetic apriori knowledge we can have is of how our minds organise the data of the senses; that organising framework is space and time, which for Kant have no mind-independent existence, but nonetheless operate uniformly in all humans. Apriori knowledge of space and time is all that remains of metaphysics as traditionally conceived. There is a reality beyond sensory data or phenomena, which he calls the realm of noumena; however, we cannot know it as it is in itself, but only as it appears to us. He allows himself to speculate that the origins of God, morality, and free will might exist in the noumenal realm, but these possibilities have to be set against its basic unknowability for humans. Although he saw himself as having disposed of metaphysics, in a sense, he has generally been regarded in retrospect as having a metaphysics of his own.
Nineteenth century philosophy was overwhelmingly influenced by Kant and his successors. Schopenhauer, Schelling, Fichte and Hegel all purveyed their own panoramic versions of German Idealism, Kant's own caution about metaphysical speculation, and refutation of idealism, having fallen by the wayside. The idealistic impulse continued into the early twentieth century with British idealists such as F. H. Bradley and J. M. E. McTaggart.
Early analytical philosophy and positivism
During the period when idealism was dominant in philosophy, science had been making great advances. The arrival of a new generation of scientifically minded philosophers led to a sharp decline in the popularity of idealism during the 1920s.
The early to mid twentieth century philosophy also saw a trend to reject metaphysical questions as meaningless. The driving force behind this tendency was the philosophy of Logical Positivism as espoused by the Vienna Circle.
At around the same time, the American pragmatists were steering a middle course between materialism and idealism. System-building metaphysics, with a fresh inspiration from science, was revived by A. N. Whitehead and Charles Hartshorne.
The forces that shaped analytical philosophy—the break with idealism, and the influence of science — were much less significant outside the English speaking world, although there was a shared turn toward language. Continental philosophy continued in a trajectory from post Kantianism.
The phenomenology of Husserl and others was intended as a collaborative project for the investigation of the features and structure of consciousness common to all humans, in line with Kant's basing his synthetic apriori on the uniform operation of consciousness. It was officially neutral with regards to ontology, but was nonetheless to spawn a number of metaphysical systems. Brentano's concept of intentionality would become widely influential, including on analytical philosophy.
Heidegger, author of Being and Time, saw himself as re-focusing on Being-qua-being, introducing the novel concept of Dasein in the process. Classing himself an existentialist, Sartre wrote an extensive study of Being and Nothingness.
The speculative realism movement marks a return to full blooded realism.
There are two fundamental aspects of everyday experience: change and persistence. Until recently, the Western philosophical tradition has arguably championed substance and persistence, with some notable exceptions however. According to process thinkers, novelty, flux and accident do matter, and sometimes they constitute the ultimate reality.
Lato sensu, process metaphysics is as old as Western philosophy, with figures such as Heraclitus, Plotinus, Duns Scotus, Leibniz, David Hume, Georg Wilhelm Friedrich Hegel, Friedrich Wilhelm Joseph von Schelling, Gustav Theodor Fechner, Friedrich Adolf Trendelenburg, Charles Renouvier, Karl Marx, Ernst Mach, Friedrich Wilhelm Nietzsche, Émile Boutroux, Henri Bergson, Samuel Alexander and Nicolas Berdyaev. It seemingly remains an open question whether major "Continental" figures such as the late Martin Heidegger, Maurice Merleau-Ponty, Gilles Deleuze, Michel Foucault, or Jacques Derrida should be included.
Stricto sensu, process metaphysics may be limited to the works of a few founding fathers: G. W. F. Hegel, Charles Sanders Peirce, William James, Henri Bergson, A. N. Whitehead, and John Dewey. From a European perspective, there was a very significant and early Whiteheadian influence on the works of outstanding scholars such as Émile Meyerson (1859–1933), Louis Couturat (1868–1914), Jean Wahl (1888–1974), Robin George Collingwood (1889–1943), Philippe Devaux (1902–1979), Hans Jonas (1903–1993), Dorothy M. Emmett (1904–2000), Maurice Merleau Ponty (1908–1961), Enzo Paci (1911–1976), Charlie Dunbar Broad (1887–1971), Wolfe Mays (1912–), Ilya Prigogine (1917–2003), Jules Vuillemin (1920–2001), Jean Ladrière (1921–), Gilles Deleuze (1925–1995), Wolfhart Pannenberg (1928–), and Reiner Wiehl (1929–2010).
Later analytical philosophy
While early analytic philosophy tended to reject metaphysical theorizing, under the influence of logical positivism, it was revived in the second half of the twentieth century. Philosophers such as David K. Lewis and David Armstrong developed elaborate theories on a range of topics such as universals, causation, possibility and necessity and abstract objects. However, the focus of analytical philosophy generally is away from the construction of all-encompassing systems and toward close analysis of individual ideas.
Among the developments that led to the revival of metaphysical theorizing were Quine's attack on the analytic-synthetic distinction, which was generally taken to undermine Carnap's distinction between existence questions internal to a framework and those external to it.
The philosophy of fiction, the problem of empty names, and the debate over existence's status as a property have all risen out of relative obscurity to become central concerns, while perennial issues such as free will, possible worlds, and the philosophy of time have had new life breathed into them.
Rejections of metaphysics
A number of individuals have suggested that much of metaphysics should be rejected. In the eighteenth century, David Hume took an extreme position, arguing that all genuine knowledge involves either mathematics or matters of fact and that metaphysics, which goes beyond these, is worthless. He concludes his Enquiry Concerning Human Understanding with the statement:
- If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.
In the 1930s, A. J. Ayer and Rudolf Carnap endorsed Hume's position; Carnap quoted the passage above. They argued that metaphysical statements are neither true nor false but meaningless since, according to their verifiability theory of meaning, a statement is meaningful only if there can be empirical evidence for or against it. Thus, while Ayer rejected the monism of Spinoza, noted above, he avoided a commitment to pluralism, the contrary position, by holding both views to be without meaning. Carnap took a similar line with the controversy over the reality of the external world.
Thirty-three years after Hume's Enquiry appeared, Immanuel Kant published his Critique of Pure Reason. Although he followed Hume in rejecting much of previous metaphysics, he argued that there was still room for some synthetic a priori knowledge, concerned with matters of fact yet obtainable independent of experience. These included fundamental structures of space, time, and causality. He also argued for the freedom of the will and the existence of "things in themselves", the ultimate (but unknowable) objects of experience.
Arguing against such rejections, the Scholastic philosopher Edward Feser has observed that Hume's critique of metaphysics, and specifically Hume's fork, is "notoriously self-refuting". Feser argues that Hume's fork itself is not a conceptual truth and is not empirically testable.
Metaphysics in science
Much recent work has been devoted to analyzing the role of metaphysics in scientific theorizing. Alexandre Koyré led this movement, declaring in his book Metaphysics and Measurement, "It is not by following experiment, but by outstripping experiment, that the scientific mind makes progress."Imre Lakatos maintained that all scientific theories have a metaphysical "hard core" essential for the generation of hypotheses and theoretical assumptions. Thus, according to Lakatos, "scientific changes are connected with vast cataclysmic metaphysical revolutions."
An example from biology of Lakatos' thesis: David Hull has argued that changes in the ontological status of the species concept have been central in the development of biological thought from Aristotle through Cuvier, Lamarck, and Darwin. Darwin's ignorance of metaphysics made it more difficult for him to respond to his critics because he could not readily grasp the ways in which their underlying metaphysical views differed from his own.
In physics, new metaphysical ideas have arisen in connection with quantum mechanics, where subatomic particles arguably do not have the same sort of individuality as the particulars with which philosophy has traditionally been concerned. Also, adherence to a deterministic metaphysics in the face of the challenge posed by the quantum-mechanical uncertainty principle led physicists such as Albert Einstein to propose alternative theories that retained determinism.A. N. Whitehead is famous for creating a metaphysics inspired by electromagnetism and special relativity.
In chemistry, Gilbert Newton Lewis addressed the nature of motion, arguing that an electron should not be said to move when it has none of the properties of motion.
Katherine Hawley notes that the metaphysics even of a widely accepted scientific theory may be challenged if it can be argued that the metaphysical presuppositions of the theory make no contribution to its predictive success.