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Metaphysics is a branch of philosophy concerned with explaining the fundamental nature of being and the world,[1] although the term is not easily defined.[2] Traditionally, metaphysics attempts to answer two basic questions in the broadest possible terms:
A person who studies metaphysics is called a metaphysicist[4] or a metaphysician.[5] The metaphysician 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.
Prior to the modern history of science, scientific questions were addressed as a part of metaphysics known as natural philosophy. The term science itself meant "knowledge" of, originating from epistemology. 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.[6]
The word "metaphysics" derives from the Greek words μετά (metá) ("beyond", "upon" or "after") and φυσικά (physiká) ("physics").[7] 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- ("beyond") 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 (phusis 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, "In Lib, Boeth. de Trin.", V, 1).
There is a widespread use of the term in current popular literature, which replicates this error, i.e. that metaphysical means spiritual non-physical: thus, "metaphysical healing" means healing by means of remedies that are not physical.[8]
Although the word "metaphysics" goes back to Aristotelean philosophy, Aristotle himself credited earlier philosophers with dealing with metaphysical questions. The first known philosopher, according to Aristotle, is Thales of Miletus, who taught that all things derive from a single first cause or Arche.
Scientific questions in ancient Greece were addressed to metaphysicians, but by the 18th century, the skeptics' How do you know? led to a new branch of philosophy called epistemology (how we know) to fill-out the metaphysics (what we know) and this led to science (Latin to know) and to the scientific method (the precision of which is still being debated). Skepticism evolved epistemology out of metaphysics. Thereafter, metaphysics denoted philosophical inquiry of a non-empirical character into the nature of existence.[6]
Metaphysics as a discipline was a central part of academic inquiry and scholarly education even before the age of Aristotle, who considered it "the Queen of Sciences." Its issues were considered no less important than the other main formal subjects of physical science, medicine, mathematics, poetics and music. Since the beginning of modern philosophy during the seventeenth century, problems that were not originally considered within the bounds of metaphysics have been added to its purview, while other problems considered metaphysical for centuries are now typically subjects of their own separate regions in philosophy, such as philosophy of religion, philosophy of mind, philosophy of perception, philosophy of language, and philosophy of science.
In some cases, subjects of metaphysical scholarship have been found to be entirely physical and natural, thus making them part of science proper (cf. the theory of Relativity).
Aristotle's Metaphysics was divided into three parts, which are now regarded as the proper branches of traditional Western metaphysics:
Universal science or first philosophy treats of "being qua being"—that is, what is basic to all science before one adds the particular details of any one science. Essentially "being qua being" may be translated as "being insofar as being goes" or as "being in terms of being." This includes topics such as causality, substance, species and elements, as well as the notions of relation, interaction, and finitude.
Most positions that can be taken with regards to any of the following questions are endorsed by one or another notable philosopher. It is often difficult to frame the questions in a non-controversial manner.
The nature of Being is a perennial topic in metaphysics. For instance, Parmenides taught that reality was a single unchanging Being. The 20th century philosopher Heidegger thought previous philosophers have lost sight of the question of Being (qua Being) in favour of the questions of beings (existing things), so that a return to the Parmenidean approach was needed. An ontological catalogue is an attempt to list the fundamental constituents of reality. The question of whether or not existence is a predicate has been discussed since the Early Modern period, not least in relation to the ontological argument for the existence of God. Existence, that something is, has been contrasted with essence, the question of what something is. Reflections on the nature of the connection and distinction between existence and essence dates back to Aristotle’s Metaphysics, and it found one of its later most influential interpretations in the ontology of the eleventh century metaphysician Avicenna (Ibn Sina) [9]. Since existence without essence seems blank, it is associated with nothingness by philosophers such as Hegel.
The world seems to contain many individual things, both physical, like apples, and abstract such as love and the number 3; the former objects are called particulars. Particulars are said to have attributes, e.g. size, shape, color, location and two particulars may have some such attributes in common. Such attributes, are also termed Universals or Properties; the nature of these, and whether they have any real existence and if so of what kind, is a long-standing issue, realism and nominalism representing opposing views.
Metaphysicians concerned with questions about universals or particulars are interested in the nature of objects and their properties, and the relationship between the two. Some, e.g. Plato, argue that properties are abstract objects, existing outside of space and time, to which particular objects bear special relations. David Armstrong holds that universals exist in time and space but only at their instantiation and their discovery is a function of science. Others maintain that particulars are a bundle or collection of properties (specifically, a bundle of properties they have).
Some philosophers endorse views according to which there are abstract objects such as numbers, or Universals. (Universals are properties that can be instantiated by multiple objects, such as redness or squareness.) Abstract objects are generally regarded as being outside of space and time, and/or as being causally inert. Mathematical objects and fictional entities and worlds are often given as examples of abstract objects. The view that there really are no abstract objects is called nominalism. Realism about such objects is exemplified by Platonism. Other positions include moderate realism, as espoused by Aristotle, and conceptualism.
The philosophy of mathematics overlaps with metaphysics because some positions are realistic in the sense that they hold that mathematical objects really exist, whether transcendentally, physically, or mentally. Platonic realism holds that mathematical entities are a transcendent realm of non-physical objects. The simplest form of mathematical empiricism claims that mathematical objects are just ordinary physical objects, i.e. that squares and the like physically exist. Plato rejected this view, among other reasons, because geometrical figures in mathematics have a perfection that no physical instantiation can capture. Modern mathematicians have developed many strange and complex mathematical structures with no counterparts in observable reality, further undermining this view. The third main form of realism holds that mathematical entities exist in the mind. However, given a materialistic conception of the mind, it does not have the capacity to literally contain the many infinities of objects in mathematics. Intuitionism, inspired by Kant, sticks with the idea that "there are no non-experienced mathematical truths". This involves rejecting as intuitionistically unacceptable anything that cannot be held in the mind or explicitly constructed. Intuitionists reject the law of the excluded middle and are suspicious of infinity, particularly of transfinite numbers.
Other positions such as formalism and fictionalism that do not attribute any existence to mathematical entities are anti-realist.
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 did not draw a distinction between this use and their model for the cosmos. However, in modern times it addresses questions about the Universe beyond the scope of physical science. 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:
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 no random, spontaneous, stochastic, mysterious, or miraculous events occur. 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 coherently reconciled. Adherents of this view include Thomas Hobbes and many modern philosophers.
Incompatibilists who accept free will but reject determinism are called Libertarians, a term not to be confused with the political sense. Robert Kane is a modern defender of this theory.
It is believed that determinism necessarily entails that humanity or individual humans have no influence on the future and its events (a position known as Fatalism). Determinists believe a human being's future depends on present and past events beyond the individual's influence.
Proponents of the Copenhagen Interpretation of quantum mechanics believe that Determinism was proven incorrect with the Heisenberg Uncertainty Principle, which states that certain corresponding physical quantities (such as position and momentum, energy and time, eigenstates of spin in non-parallel directions) cannot be determined simultaneously to an arbitrarily small error. For example, the more precisely a particle's position is measured, the less precisely one can know its momentum from the same measurement. If the momentum is more precisely measured to account for this, the uncertainty in the position will rise. This counter-intuitive result is a consequence of the wave-like behavior of subatomic particles. The Heisenberg Uncertainty Principle is thus a statement providing an inverse correlation between the uncertainties of certain corresponding measurements.
This result has led physicists to develop Quantum field theory and use the "wave function" to describe physical systems. While differential equations govern the evolution of the wave function, the wave function itself only specifies the probability of an event to be measured. Thus, while the evolution of the wave function is deterministic, the history of a particle is not. The widely held view amongst physicists regards the apparent determinism of Newtonian dynamics, for example, as the limiting behavior of inherently probabilistic phenomena. Classical physics is then considered a convenient and sufficiently accurate description of nature only at macroscopic scales. A large enough scale is usually thought of as much larger than the Compton wavelength of the object considered.
Alternative interpretations of quantum theory, such as de Broglie–Bohm theory and the many-worlds interpretation, provide a deterministic theoretical framework that is consistent with empirical observation of quantum mechanical phenomena. Notable physicists, such as Albert Einstein and Erwin Schrödinger, never believed that Quantum Mechanics was a fundamental theory. Einstein is famous for believing that while Quantum Mechanics provided good experimental predictions, it should ultimately be replaced with a deterministic theory that could account for the seemingly probabilistic nature of the atomic regime. In a letter to Max Born, Einstein wrote, "Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the 'old one'. I, at any rate, am convinced that He does not throw dice."[10] This quote is commonly paraphrased "God does not play dice" or "God does not play dice with the universe", and other slight variants.
Though there remains skepticism, there is a scientific consensus that the laws of Quantum Mechanics correctly describe the universe up to current experimental bounds, and that it is necessary for the very consistency of the subject that our observations be probabilistic in nature (see Bell's Theorem).
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.
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 that holds there is a single universal substance or principles. 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, Douglas Hofstadter, Jerry Fodor, David Lewis, Thomas Nagel, Hilary Putnam, John Searle, John Smart, Ludwig Wittgenstein, and Fred Alan Wolf.
Metaphysicians 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 like 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; that is, we could not imagine it to be otherwise. 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."
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), and whether the Divine intervenes directly 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.[11]
In Book XI of the Confessions, Saint Augustine of Hippo asked the fundamental question about the nature of time. A traditional realist position in ontology is that time and space have existence apart from the human mind. Idealists, including Kant, claim that space and time are mental constructs used to organize perceptions, or are otherwise surreal.
Suppose that one is sitting at a table, with an apple in front of him or her; the apple exists in space and in time, but what does this statement indicate? Could it be said, for example, that space is like an invisible three-dimensional grid in which the apple is positioned? Suppose the apple, and all physical objects in the universe, were removed from existence entirely. Would space as an "invisible grid" still exist? René Descartes and Leibniz believed it would not, arguing that without physical objects, "space" would be meaningless because space is the framework upon which we understand how physical objects are related to each other. Newton, on the other hand, argued for an absolute "container" space. The pendulum swung back to relational space with Einstein and Ernst Mach.
While the absolute/relative debate, and the realism debate are equally applicable to time and space, time presents some special problems of its own. The flow of time has been denied in ancient times by Parmenides and more recently by J. M. E. McTaggart in his paper The Unreality of Time.
The direction of time, also known as "time's arrow", is also a puzzle, although physics is now driving the debate rather than philosophy. It appears that fundamental laws are time-reversible and the arrow of time must be an "emergent" phenomenon, perhaps explained by a statistical understanding of thermodynamic entropy.
Common-sense tells us that objects persist across time, that there is some sense in which you are the same person you were yesterday, in which the oak is the same as the acorn, in which you perhaps even can step into the same river twice. Philosophers have developed two rival theories for how this happens, called "endurantism" and "perdurantism". Broadly speaking, endurantists hold that a whole object exists at each moment of its history, and the same object exists at each moment. Perdurantists believe that objects are four-dimensional entities made up of a series of temporal parts like the frames of a movie.
The first known philosopher, according to Aristotle, is Thales of Miletus. Rejecting mythological and divine explanations, he sought for 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, rejecting the epistemological validity of sense experience, and instead 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.
Democritus and his teacher Leucippus, are known for formulating an atomic theory for the cosmos.[13] They are considered forerunners of the scientific method.
Socrates is known for his dialectic or questioning approach to philosophy rather than a positive metaphysical doctrine. His pupil, Plato is famous for his theory of forms (which he confusingly places in the mouth of Socrates in the dialogues he wrote to expound it). Platonic realism (also considered a form of idealism[14]) is principally a solution to the problem of universals; what particular horses have in common is that they partake in the Form of the Horse. The theory has a number of other aspects:
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 can exist uninstantiated in visible things, Aristotelean essences "indwell" in particulars.
Potentiality and Actuality[15] are principles of a dichotomy which Aristotle used throughout his philosophical works to analyze motion, causality and other issues.
The Aristotelean 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.
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 Aristotelean 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.
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.
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 Berkely 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 sceptical empiricism, not to forget the burgeoning science of his day. Like 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 apriori) but is nonetheless about reality (synthetic). Inasmuch as it is about reality, it is unlike 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.
19th 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 20th century with British idealists such as F. H. Bradley and J. M. E. McTaggart.
Followers of Karl Marx took Hegel's dialectic view of history and re-fashioned it as materialism.
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.
Analytical philosophy was spearheaded by Bertrand Russell and G. E. Moore. Russell and William James tried to compromise between idealism and materialism with the theory of neutral monism.
The early to mid 20th century philosophy also saw a trend to reject metaphysical questions as meaningless. The driving force behind this tendency was the unabashedly materialist philosophy of Logical Positivism as espoused by the Vienna Circle. See "The Value and Future of Metaphysics" below.
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.
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 Kellogg 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 is generally away from the construction of all-encompassing systems and towards 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.[16]
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.[17][18]
A number of individuals have suggested that metaphysics as a whole should be rejected.
David Hume argued with his empiricist principle that all knowledge involves either relations of ideas or matters of fact:
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.— David Hume, An Enquiry Concerning Human Understanding
However, Hume's assertion may be self-defeating if it itself is not self-evident or empirically verifiable.[19]
Immanuel Kant prescribed a limited role to the subject and argued against knowledge progressing beyond the world of our representations, except to knowledge that the noumena exist:
...though we cannot know these objects as things in themselves, we must yet be in a position at least to think them as things in themselves; otherwise we should be landed in the absurd conclusion that there can be appearance without anything that appears.— Immanuel Kant, Critique of Pure Reason pp. Bxxvi-xxvii
Proceeding from Kant's statement about antinomy, A.J. Ayer in Language, Truth and Logic using the verifiability theory of meaning concluded that metaphysical propositions were neither true nor false but strictly meaningless, as were religious views. However, Karl Popper argued that metaphysical statements are not meaningless statements, but rather not fallible, testable or provable statements[20] i.e. neither empirical observations nor logical arguments could falsify metaphysical statements to show them to be true or false. Hence, a metaphysical statement usually implies an idea about the world or about the universe, which may be reasonable but is ultimately not empirically testable.
Rudolf Carnap, in his book Philosophy and Logical Syntax, used the concept of verifiability to reject metaphysics.
Metaphysicians cannot avoid making their statements nonverifiable, because if they made them verifiable, the decision about the truth or falsehood of their doctrines would depend upon experience and therefore belong to the region of empirical science. This consequence they wish to avoid, because they pretend to teach knowledge which is of a higher level than that of empirical science. Thus they are compelled to cut all connection between their statements and experience; and precisely by this procedure they deprive them of any sense.— Rudolf Carnap
John Locke, a founder of empiricism, expressed that most of the doctrine of innate ideas in the metaphysics, such as Cartesian dualism and the Platonic realm were ridiculous and nonsensical.[21]
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