Metaphysical Mind-Body Relations

Abstract

Metaphysical mind-body relations name the philosophical positions on how conscious mental states relate to physical brain processes — a question the National Library of Medicine's Medical Subject Headings classifies under mental processes, though it concerns what the mind is rather than a process it performs. This article maps the principal answers: dualism, on which mind and matter differ in kind, and physicalism, ranging from type-identity theory through functionalism to eliminative materialism. It then sets out the challenges that keep the problem open — Nagel's subjective character, Jackson's knowledge argument, Levine's explanatory gap, Chalmers' hard problem, and Kim's causal-exclusion worry about mental causation. A closing section shows how the empirical science of consciousness inherits rather than dissolves the metaphysics. Three demonstrations let the reader locate the positions on a map, watch a global-workspace ignition curve, and test multiple realizability.

Keywords: mind-body problem, consciousness, physicalism, qualia

Every other topic in this branch of the classification is an operation the mind performs — encoding a stimulus, retrieving a memory, selecting a response. The mind-body problem is different: it does not ask what the mind does but what the mind is, and how whatever it is stands to the three-pound organ that appears to produce it. The question is old, but it is not idle, because the science of the mind assumes an answer whether or not it states one. A reaction-time experiment that treats a decision as a brain event, a neuroimaging study that hunts for the signature of awareness, a clinician who infers a patient's inner state from cortical activity — each takes for granted some relation between the mental and the physical. This article makes that relation explicit, sets out the positions philosophers have defended, and traces why the problem has proved so resistant that a working scientist cannot simply wait for it to be settled.

The Mind-Body Problem

The problem begins with an apparent clash between two things each hard to deny. On one side, conscious experiences seem to have features no physical description captures: the redness of red, the ache of a pain, what it is like to hear a cello. On the other, those experiences depend so tightly on the brain that damaging tissue, administering a drug, or stimulating a region reliably changes them, which makes the mental look like a physical affair. The mind-body problem is the task of holding both facts together: saying what conscious states are, such that they can have their vivid felt character and stand in the intimate dependence on the brain that the evidence shows.

Answers sort along two axes, and the first demonstration lays them on exactly these. The first axis asks how many kinds of thing there are: monism, that there is ultimately one kind of stuff, against dualism, that mind and matter are two. The second asks, within monism, whether mental facts reduce to physical facts — whether they are nothing over and above them — or float free as something additional. Substance dualism sits at one extreme, holding mind and body to be separate substances; type-identity theory sits at the other, holding each mental state to be literally a brain state; and functionalism, property dualism, and eliminative materialism take intermediate ground. Table 1 previews the positions and the single difficulty that dogs each, which the sections below develop in turn.

Dualism

Dualism is the oldest answer and the one closest to common sense: mind and matter are fundamentally different. Its canonical form is the substance dualism associated with Descartes, on which the mind is a thinking, non-extended substance and the body an extended, non-thinking one, each able in principle to exist without the other (Descartes, 1641/1984). The view honours the felt gulf between an inner life and the physical world, and it explains why introspection reveals nothing bodily about a thought. Its fatal difficulty is causal. If deciding to raise an arm is an event in a non-physical mind, and the arm rising is a chain of physical events in nerve and muscle, then the two must interact — yet no one has made intelligible how a substance with no physical properties could impart motion to one that has them, without violating the causal closure of the physical world. This interaction problem has dogged substance dualism since Princess Elisabeth of Bohemia pressed it on Descartes directly in their correspondence, asking how a wholly immaterial mind could ever move a material body, and it is the main reason the position has few defenders among those who take physics seriously (Shapiro, 2007).

Property dualism is the more resilient descendant. It concedes that there is only one kind of substance, the physical, but denies that all its properties are physical: conscious experiences, it holds, are further properties that arise from but do not reduce to the brain's physical properties. On this view a complete physics would still omit something — the felt quality of experience — because that quality is a genuinely additional feature of the world. Property dualism avoids the crudest form of the interaction problem, since it posits no separate substance, but it inherits a subtler one. If phenomenal properties are non-physical yet the physical world is causally closed, then those properties seem unable to do anything: the behaviour they accompany is already fully caused by the physical substrate, leaving experience a causally idle by-product. That consequence, epiphenomenalism, is a cost the property dualist must either pay or explain away, and it recurs below in Jackson's knowledge argument.

Physicalism

Physicalism is the thesis that the mental is not something over and above the physical: mental states either are physical states or are wholly realized by them. Its first modern form was the type-identity theory advanced by U. T. Place and J. J. C. Smart, who proposed that consciousness is quite literally a brain process, in the same way that lightning is an electrical discharge or water is H₂O — not correlated with it, but identical to it (Place, 1956; Smart, 1959). Identity theory is austere and clear: it dissolves the interaction problem at a stroke, because there are not two things to interact, and it locates the mind squarely within the science of the brain. Its vulnerability is generality. If pain is identical to a specific brain state — say the firing of C-fibres — then any creature lacking that exact state cannot be in pain, which seems false of an octopus, a bird, or a conceivable silicon system that behaves as though in pain.

That objection, the multiple-realizability argument, was pressed by Hilary Putnam and sharpened by Jerry Fodor into the founding case for functionalism (Putnam, 1967; Fodor, 1974). Putnam's point was that the same mental state can be realized in physically very different systems, so it cannot be identical to any one of them; what a mental state is, on the functionalist proposal that followed, is a matter of the causal role it plays — the pattern of inputs that produce it and outputs it produces — abstracted away from the stuff that fills the role. Pain is whatever state is typically caused by bodily damage and typically causes wincing, aversion, and the belief that one is hurt, whether that state is realized in neurons, in the distributed ganglia of an octopus, or in a machine. Fodor generalised the moral to a defence of the special sciences: psychology's kinds are multiply realizable, so they cannot be reduced to physics even though every instance of them is physical. The third demonstration lets the reader vary the substrate and watch the type-identity claim fail exactly when a second, physically different realizer shares the state.

At the far end of the physicalist spectrum lies eliminative materialism, argued most forcefully by Paul Churchland, which holds that the ordinary mental categories — belief, desire, and the felt qualities of experience — belong to a folk theory so defective that a mature neuroscience will not reduce them but replace them, as chemistry replaced phlogiston rather than identifying it with something real (Churchland, 1981). Eliminativism is the boldest bet in the debate and the least intuitive: critics charge that denying beliefs is self-undermining, since asserting the view seems to express one. Whether one finds it a reductio of hard-nosed physicalism or a promissory note on a future science, it marks the position's outer limit — and, with the four positions above, completes the map that the first demonstration renders. Selecting a position there reveals its core claim, a representative proponent, and the standard objection, the same four rows Table 1 previews.

The Hard Problem and the Explanatory Gap

Even a thoroughgoing physicalism faces a residue that will not go quietly, and three linked arguments circle it. Thomas Nagel put it first and most vividly: an organism is conscious if there is something it is like to be that organism, and this subjective character — what it is like, from the inside, to be a bat navigating by sonar — is precisely what an objective, third-person physical account seems constitutionally unable to capture, because it is tied to a particular point of view (Nagel, 1974). No amount of knowledge about echolocation tells us what the bat's experience is like for the bat. Figure 1 renders the shape of the difficulty: a body of third-person physical facts on one side, first-person phenomenal facts on the other, and a gap between them that the physical facts do not seem to close.

Frank Jackson turned the intuition into an argument. Imagine Mary, a scientist who knows every physical fact about colour vision but has lived her whole life in a black-and-white room; when she first sees red, Jackson argued, she learns something new — what red looks like — so there was a fact about experience the complete physical story left out (Jackson, 1982). Pressed on whether Mary gains new knowledge or merely a new ability, Jackson defended the knowledge reading, insisting that what Mary lacked was information, not a skill (Jackson, 1986). Joseph Levine gave the residue its enduring name: between a physical description of a state and its felt quality there is an explanatory gap, such that even a true identity — pain is C-fibre firing — does not explain why that state feels the way it does, in the way that water's being H₂O explains why it boils when it does (Levine, 1983). David Chalmers then drew the distinction that organises the modern debate: the easy problems of consciousness are the functions — discrimination, integration, reportability — that a mechanistic account can in principle explain, while the hard problem is why any of that functioning is accompanied by experience at all (Chalmers, 1995). He later pressed the intuition into a direct argument against physicalism: a philosophical zombie — a being molecule-for-molecule identical to a conscious person yet wholly without experience — seems conceivable, and if such a creature is so much as possible, then consciousness does not logically supervene on the physical and no complete physical description of the brain could entail the facts about experience (Chalmers, 1996). Ned Block sharpened what is at issue by separating two things the word consciousness runs together: access consciousness, information poised for use in reasoning and report, and phenomenal consciousness, the felt quality itself — a distinction that lets one grant a full functional account of access while insisting the phenomenal remains untouched (Block, 1995).

Not everyone grants the residue. Daniel Dennett argued that the hard problem is an artefact of taking introspective report at face value, and that once every function is explained — every discrimination, reaction, and report a subject can make — nothing genuine remains to be explained; the sense of an unbridgeable gap is, on his deflationary view, a cognitive illusion rather than a datum. The disagreement is not over the facts of brain function, which both sides accept, but over whether a complete functional account would ipso facto be a complete account of consciousness — the same functional strategy the global-workspace theories in the next section pursue. That is the fault line the empirical programme must build across.

Where Dennett deflates the residue, a rival response takes it so seriously that it revises what the physical is. Panpsychism holds that experience, or something proto-experiential, is a fundamental and ubiquitous feature of the physical world rather than a late arrival that emerges only in complex brains. Galen Strawson pressed the case as a demand of physicalism itself: if experiential character is real and the physical is all there is, then taking physicalism seriously entails that some experiential nature runs all the way down, because genuine experience cannot be conjured from ingredients that are wholly and radically non-experiential (Strawson, 2006). Philip Goff has developed the position as a form of Russellian monism, on which physics describes only the causal and structural profile of matter — what it does — and leaves its intrinsic nature open, a gap that consciousness is placed to fill (Goff, 2017). The attraction is that panpsychism promises to close the explanatory gap without either dualism's causal embarrassment or physicalism's leftover residue; its standing difficulty is the combination problem — that it is no clearer how a multitude of micro-experiences could constitute a single unified subject than how neurons could — so the view relocates the hard problem rather than obviously dissolving it.

Supervenience and Mental Causation

Most physicalists who reject both dualism and eliminativism occupy a middle position, nonreductive physicalism, expressed through the notion of supervenience: mental properties supervene on physical ones in that there can be no difference in the mental without a difference in the physical, so fixing every physical fact fixes every mental fact, yet the mental is not thereby reduced to the physical. Supervenience promised to secure physicalism — no free-floating minds — while preserving the autonomy of psychology that multiple realizability seemed to demand. Jaegwon Kim argued that the promise cannot be kept. His causal-exclusion argument runs as follows: if a mental event causes a physical effect, and the physical world is causally closed so that the effect already has a sufficient physical cause, then either the mental cause is identical to that physical cause — which is reduction, the very thing nonreductive physicalism denies — or the effect is overdetermined by two sufficient causes, or the mental cause does no work and is excluded (Kim, 1989). The nonreductive physicalist wants mental states to be causally efficacious and irreducible and the physical to be closed, and Kim's point is that the three cannot all hold. The argument does not by itself decide the issue, but it sets the bill: any account on which the mental is more than the physical must explain how the mental earns its causal keep, which is the same difficulty property dualism met as epiphenomenalism.

From Metaphysics to Neuroscience

None of this is why a philosophy of mind topic sits in a classification of mental processes. The connection is that the empirical science of consciousness, a live part of cognitive neuroscience, is built on a metaphysical bridge whether it says so or not. The programme's organising goal is to find the neural correlates of consciousness — the minimal neural mechanisms jointly sufficient for a specific conscious experience — and Christof Koch and colleagues have pressed both the progress and the problem: locating a correlate tells us where and when, but a correlate is not yet an explanation of why that activity is experienced (Koch, Massimini, Boly, & Tononi, 2016). Even the anatomy is contested, with evidence and argument advanced for the neural substrate lying toward the back of the cortex rather than the front (Boly et al., 2017).

Two theories go further and propose bridge principles. Global workspace theory, developed by Bernard Baars and given a neuronal form by Stanislas Dehaene and Jean-Pierre Changeux, holds that a mental content becomes conscious when it is broadcast across a network of long-range cortical neurons — when it ignites and is made available to the whole cognitive system for report, memory, and control (Baars, 2005; Dehaene & Changeux, 2011). Ignition is nonlinear: below a threshold of stimulus strength a stimulus is processed but stays local and unreported, and above it activity amplifies into a global broadcast, so reportability switches within a narrow band. The second demonstration models that switch as a logistic curve and is the basis of the Worked Example.

Integrated information theory, due to Giulio Tononi and developed with Koch, takes the opposite direction of explanation: it starts from the felt properties of experience and posits that consciousness is integrated information, quantified as Φ, a measure of how much a system's cause-effect structure is irreducible to its parts (Tononi, Boly, Massimini, & Koch, 2016).

That these are rival bridges, not settled findings, is itself a live methodological topic. Anil Seth and Tim Bayne have surveyed the major theories and stressed how differently they carve the problem and how hard they are to adjudicate empirically (Seth & Bayne, 2022), while Adrien Doerig and colleagues have argued that some theories face structural obstacles to being tested at all — that their commitments make certain predictions unfalsifiable in principle (Doerig, Schurger, & Herzog, 2021). Others try to dissolve the hard problem from within physicalism: Mark Solms has argued that the free-energy principle, by grounding consciousness in an organism's imperative to regulate its own states, can meet the hard problem rather than sidestep it (Solms, 2018). The point for this article is not which theory is right but that each takes a stance on the metaphysics: a correlate presupposes the mental is at least lawfully tied to the physical, a bridge principle presupposes a specific form of that tie, and the hard problem is the standing question of whether any such principle can be more than a correlation dressed as an explanation.

Worked Example

The ignition idea can be made quantitative, and the second demonstration reproduces the calculation. Model the probability that a stimulus ignites the global workspace and becomes reportable as a logistic function of its strength s: P(s) = 1 / (1 + e^(−k(s − θ))), with threshold θ = 0.5 and steepness k = 12, where s runs from 0 (no signal) to 1 (maximal). The logistic form captures the theory's central claim that consciousness is all-or-none at threshold: the curve is shallow far from θ and nearly vertical at it.

Take two equal nudges to stimulus strength, one straddling the threshold and one far below it. Near threshold, raising s from 0.45 to 0.55 raises P from 1 / (1 + e^(−12·(−0.05))) = 0.354 to 1 / (1 + e^(−12·0.05)) = 0.646 — a jump of 0.292, from 35% to 65% reportable, produced by a change of just 0.10 in strength. The same 0.10 change far from threshold, from 0.10 to 0.20, moves P from 0.008 to 0.027 — a change of 0.019, from under 1% to under 3%. The identical physical increment thus decides consciousness in one place and does almost nothing in another. That nonlinearity is the quantitative content of ignition: it is why global workspace theory predicts a sharp perceptual threshold rather than a gradual fade, and why a small difference in stimulus strength near threshold flips a stimulus from unseen to seen. The model is illustrative, not a fit to data, but it shows how a bridge principle turns a metaphysical proposal — content becomes conscious when broadcast — into a testable curve.

Discussion

The mind-body problem has the peculiar status of being both perennial and practical. It is perennial because the core difficulty — that the felt quality of experience seems to slip through every objective net — has survived every reformulation, from Descartes' substances to Chalmers' hard problem, and because each proposed solution buys its advantages at a recognisable price. Dualism honours the datum of experience but cannot explain causation; identity theory secures causation but founders on multiple realizability; functionalism handles realizability but faces the charge that it omits the very thing to be explained; eliminativism escapes the residue by denying it, at a cost most will not pay; and nonreductive physicalism, the popular compromise, meets Kim's demand to say how the mental could matter causally without collapsing into reduction (Nagel, 1974; Kim, 1989). The map has no uncontested centre.

It is practical because the science of the mind cannot proceed in full agnosticism. A study that seeks the neural correlate of an experience has already assumed the experience is lawfully tied to neural activity; a theory that says content becomes conscious when it ignites, or when its integrated information crosses a bound, has already committed to a bridge principle of a particular shape (Koch et al., 2016; Tononi et al., 2016; Seth & Bayne, 2022). What the empirical programme has not done, and what its most careful practitioners concede it has not done, is close the explanatory gap: showing that a neural state is sufficient for an experience is not yet showing why it feels as it does (Levine, 1983). For the working cognitive scientist the honest position is therefore a methodological one — treat the mental as tied to the physical closely enough to study, while holding open the question of what that tie ultimately is. Understanding consciousness as a natural phenomenon and understanding it as a felt one are not yet the same achievement, and knowing which one a given result delivers is itself part of doing the science well.

Common Misconceptions

The mind-body problem is a religious or mystical question, not a scientific one.

The problem is posed and debated in wholly naturalistic terms, and it constrains empirical work directly: the design of a study seeking the neural basis of awareness, and the interpretation of its results, both turn on which relation between mental and physical one assumes. It is a live question inside cognitive neuroscience, not a matter left to faith (Chalmers, 1995; Seth & Bayne, 2022).

Finding the neural correlates of consciousness would solve the mind-body problem.

A neural correlate identifies activity that is present when an experience is, but a correlation is not an explanation of why that activity is accompanied by experience at all. This is the explanatory gap: locating the mechanism leaves untouched the question the hard problem poses (Koch, Massimini, Boly, & Tononi, 2016; Levine, 1983).

Because a mental state can be realized in different systems, the mind cannot be physical.

Multiple realizability tells against type-identity theory — the claim that a mental state is one specific brain state — but it is the founding argument for functionalism, a physicalist view on which every instance of the state is physical even though the type is not tied to one substrate. The moral is against reduction to a single physical kind, not against physicalism (Putnam, 1967; Fodor, 1974).

Glossary

Access consciousness.

Block's term for mental content that is poised for use in reasoning, report, and the control of action, defined by its functional availability rather than its felt quality.

Combination problem.

The central objection to panpsychism: the difficulty of explaining how many fundamental micro-experiences could combine to constitute the single, unified subject of a human experience.

Dualism.

The view that mind and matter are fundamentally distinct; in its substance form two kinds of thing, in its property form one substance with irreducibly non-physical mental properties.

Eliminative materialism.

The physicalist view that the ordinary categories of mind — belief, desire, qualia — belong to a false folk theory that a mature neuroscience will replace rather than reduce.

Epiphenomenalism.

The position that conscious properties are caused by physical processes but themselves cause nothing, arising as causally idle by-products of brain activity.

Explanatory gap.

Levine's term for the residue left when a physical account of a mental state fails to explain why that state has the felt quality it does, even if the account is true.

Functionalism.

The view that a mental state is defined by its causal role — the inputs that produce it and the outputs it causes — and so can be realized in any system that fills the role, whatever its substrate.

Global workspace theory.

The theory that a content becomes conscious when it is broadcast across a network of long-range cortical neurons — ignited — and thereby made available to the whole cognitive system.

Hard problem of consciousness.

Chalmers' name for the question of why physical processing is accompanied by subjective experience at all, as distinct from the easy problems of explaining cognitive functions.

Identity theory.

The physicalist view that each type of mental state is identical to a type of brain state, as lightning is identical to an electrical discharge; also called type-identity theory.

Integrated information theory.

Tononi's theory that consciousness is integrated information, quantified as Φ, the degree to which a system's cause-effect structure is irreducible to that of its parts.

Knowledge argument.

Jackson's thought experiment in which Mary, who knows every physical fact about colour vision but has never seen colour, learns something new on first seeing red — suggesting some fact about experience is non-physical.

Mind-body problem.

The problem of saying how conscious mental states relate to physical brain states, reconciling the felt character of experience with its intimate dependence on the brain.

Multiple realizability.

The claim that one and the same mental state can be realized by physically diverse systems, used to argue against type-identity theory and in favour of functionalism.

Neural correlate of consciousness.

The minimal set of neural mechanisms jointly sufficient for a specific conscious experience; the central empirical target of the science of consciousness.

Panpsychism.

The view that experience, or a proto-experiential nature, is a fundamental and ubiquitous feature of the physical world; in its Russellian form it takes consciousness to be the intrinsic nature that physics leaves unspecified.

Phenomenal consciousness.

Block's term for the felt, qualitative character of experience itself — what it is like to undergo a state — as distinct from its functional accessibility.

Philosophical zombie.

A hypothetical being physically identical to a conscious person yet wholly lacking experience, used in Chalmers' conceivability argument: if such a creature is possible, consciousness does not logically supervene on the physical.

Physicalism.

The thesis that everything, including the mental, is physical or is wholly realized by the physical; also called materialism.

Property dualism.

The view that there is only one kind of substance but that conscious properties are a further, non-physical kind of property that does not reduce to physical properties.

Qualia.

The introspectively accessible qualitative properties of experience — the redness of red, the ache of a pain — whose apparent resistance to physical explanation drives the hard problem.

Supervenience.

The relation in which mental properties depend on physical ones such that no mental difference is possible without a physical difference, invoked to state physicalism without asserting reduction.

Key Researchers

David J. Chalmers. Professor of Philosophy and Neural Science at New York University; he framed the hard problem of consciousness, distinguishing the felt quality of experience from the functions a mechanistic account can explain, and defends a form of property dualism. Faculty Page - Google Scholar - Wikipedia

Patricia S. Churchland. Professor Emerita of Philosophy at the University of California, San Diego; she developed neurophilosophy and, with Paul Churchland, the eliminative-materialist program that treats folk psychology as a theory ripe for replacement by neuroscience. Faculty Page - Profile - Google Scholar - Wikipedia

Daniel C. Dennett (1942-2024). University Professor of Philosophy at Tufts University; he advanced a functionalist, deflationary account of consciousness, arguing that explaining every cognitive function leaves no further hard problem to solve. Memorial - Wikipedia

Frank Jackson. Emeritus Professor in the School of Philosophy at the Australian National University; he formulated the knowledge argument through the case of Mary the colour scientist, pressing the claim that physical knowledge leaves out what experience is like. ORCID - Faculty Page - Google Scholar - Wikipedia

Jaegwon Kim (1934-2019). Professor of Philosophy at Brown University; he developed the framework of supervenience and the causal-exclusion argument, pressing nonreductive physicalism to say how mental states can be causally efficacious. Faculty Page - Google Scholar - Wikipedia - Wikidata

Thomas Nagel. University Professor of Philosophy and Law Emeritus at New York University; his question of what it is like to be a bat crystallised the claim that the subjective character of experience resists objective, physical reduction. Faculty Page - Wikipedia - Wikidata

Hilary Putnam (1926-2016). Cogan University Professor Emeritus at Harvard University; he introduced machine-state functionalism and the multiple-realizability argument against type-identity theory, later becoming a searching critic of the functionalism he founded. Faculty Page - Memorial - Wikipedia

Frequently Asked Questions

What is the mind-body problem?
It is the philosophical problem of how conscious mental states relate to physical brain states. It asks how experiences can have their vivid felt character and at the same time depend so tightly on the brain that changing the brain changes them (Chalmers, 1995).

What is the difference between dualism and physicalism?
Dualism holds that mind and matter are fundamentally distinct, whether as two substances or as one substance with irreducibly non-physical mental properties. Physicalism holds that mental states either are physical states or are wholly realized by them, with no mental ingredient over and above the physical.

What is the hard problem of consciousness?
It is the question of why physical processing in the brain is accompanied by subjective experience at all. Chalmers separated it from the easy problems, which concern explaining cognitive functions such as discrimination and report, and which a mechanistic account can in principle handle (Chalmers, 1995).

What is the knowledge argument, or Mary's room?
It is Jackson's thought experiment about Mary, who knows every physical fact about colour vision but has never seen colour. When she first sees red she seems to learn something new, which suggests that some fact about experience was left out of the complete physical account (Jackson, 1982).

What is multiple realizability?
It is the claim that one and the same mental state can be realized in physically very different systems, so pain need not be a single brain state. It argues against type-identity theory and supports functionalism, which defines a state by its causal role rather than its substrate (Putnam, 1967; Fodor, 1974).

What is the explanatory gap?
It is Levine's name for the residue left when a physical account of a mental state does not explain why that state feels the way it does. Even a true identity between a mental state and a brain state, he argued, leaves the felt quality unexplained (Levine, 1983).

Does neuroscience solve the mind-body problem?
Not on its own. Neuroscience can find the neural correlates of consciousness and propose bridge principles such as global broadcast or integrated information, but a correlate is not yet an explanation of why the activity is experienced, so the metaphysical question remains open (Koch, Massimini, Boly, & Tononi, 2016; Seth & Bayne, 2022).

Why does a philosophy topic appear under mental processes in MeSH?
The Medical Subject Headings classify the topic there because it concerns the relation of mind to brain, which bears directly on how mental processes are studied. It is an indexing placement for the biomedical literature, not a claim that the metaphysical question is itself a process the mind carries out.

References