The Causal Closure of Physics: An Explanation and Critique

© Kile Jones 2006

Abstract:  Is the physical world causally closed?  Can something immaterial have any causal role within physics?  This paper seeks to answer these questions by explaining the theory of Causal Closure.  Causal Closure says that nothing immaterial can have any causal efficacy upon the material world.  Physicalists have long held this position and have used it as an argument against Dualism, but does it hold?  The hope of this paper is that we may better understand the arguments for and against Causal Closure in order to discover a cogent philosophy of science. 

Keywords:  Causal Closure, Physicalism, Dualism, Completeness, Mental States, Qualia.

The claim that the physical world is a causally closed system is encouraged by the advance of physical science, though, of course, it goes far beyond what science has been able to establish and probably what it can ever establish.[1]

            Today, within the fields of natural science, when an effect comes about its cause is assumed to be strictly physical.  We in science believe that physical events are caused by prior physical arrangements and therefore can be reductively explained down to microphysical processes.  Yet many do not end here.  Many philosophers of science go beyond the observational claims that physical events have solely physical causes, to the theory that the physical world is causally closed.  Physicalists, for instance, have long been supporters of Causal Closure and seek to provide warrant for this philosophical step.  On the other hand modern Dualists argue that this step is unwarranted and unnecessarily goes beyond what science teaches us, after all, they say, does the theory of Causal Closure truly come from just reading the data off of physical processes?  In this paper I will examine the theory of the Causal Closure of Physics and provide arguments against it in hopes that a better and more balanced physical theory can be captured.

Defining Causal Closure               

            Before proceeding on this journey the theory of Causal Closure (now on referred to as CC) must be defined.[2]  Eric Marcus says the theory of CC is that “Nothing non-physical can affect the physical”[3]; Tim Crane defines CC (he uses the terminology Causal Completeness) as the idea that “fixing the physical causes fixes all physical effects”[4]; and Robert Bishop classifies CC as the theory “that all physical effects are fully determined by fundamental laws and prior physical events”.[5]

            What each of these definitions are hitting at is that CC is the theory that there can only be physical explanations for physical events and that nothing non-physical can play any causal role within physics.[6]  When we see a physical effect, according to CC, we must not only assume that it has a prior physical cause but that it can only have a physical cause.    We should not even consider the idea that anything non-physical caused any physical event, for to do so would deny CC.  Thus what is entailed by CC is causal exclusion from anything outside of the closed physical system of the universe leaving any causation as strictly physical causation.  Some scholars feel that a Soft Causal Closure (or Completeness) does not rule out the possibility of non-physical causes but only states that we don’t have to go outside of the physical to supply a sufficient cause of an event.[7]   This is a position that needs qualification and if held must reject any Closure itself.  To explain these ideas more fully I will now turn to Completeness versus Closure.

Completeness vs. Closure

            In his article “Mental Causation in a Physical World”, Eric Marcus makes a distinction between Causal Completeness and Causal Closure.  He distinguishes the two: “The view that we don’t need to depart from physical explanations to explain physical events is Completeness; the view that we necessarily go wrong in departing from physical explanations to explain physical events is Closure”.[8]  What Marcus is trying to get at is that physics can give a completely adequate and sufficient explanation for events and still be open to non-physical causes (Completeness) and that this view is not synonymous with Closure which does not allow for non-physical causes.  For Marcus as well as Barbara Montero, the Completeness of physics does not rule out non-physical causation but only modifies it to say that we can always have a completely physical explanation for physical effects.  The question is to which degree non-physical causes

actually cause anything.[9]  What I find difficult is the terminology employed to speak of this dualistic philosophy of causality.  If we are going to say that non-physical entities

can affect the physical realm then we should not label it “Closure”, for physics, in this view, is not actually closed, rather, we should stick with labeling it “Completeness” for the sake of clarity.

            There is still a question left for Completeness.  How can the explanation of a physical effect be complete if we are leaving out part of the cause?  If non-physical entities are playing a contributing role in causality then are our physical explanations truly complete?  Granted that we can give an adequate and sufficient physical explanation of physical effects, what do non-physical causes actually cause?  It seems that we are left with an irrelevant non-physical cause, one to which we do not need to point to in order to account for any causation.  Yet if non-physical entities are truly causally efficacious then we have overdetermination, which must be explained.  Since overdetermination is not the topic of this paper I will leave it here, suffice it to say that both Dualism and Physicalism have the same difficulty with overdetermination.[10]  My point here is that the Completeness of Physics does leave room for non-physical causes but needs to qualify them and their causal agency.

 What Entails Causal Closure

            Now that CC has been defined we must ask ourselves what holding to CC leads us to believe about the world.  Firstly, if we believe in CC we are Physicalists.  By Physicalists I mean those who believe that this world of ours is only physical.  Jaegwon Kim defines modern Physicalism as “the idea that all things that exist in this world are bits of matter and structures aggregated out of bits of matter, all behaving in accordance with laws of physics, and that any phenomena of the world can be physically explained if it can be explained at all”.[11]  What Physicalism says, as Kim notes, is that all there is in this world are “bits of matter” and nothing else.  Therefore CC rules out any Dualism, Immanental Theism, Polytheism, or Pantheism[12], for all of these worldviews admit to interactions between non-physical entities and the physical world.

            Secondly, if we hold to CC we are left explaining what the mind is.  This leaves you with either Reductive (Identity) or NonReductive Physicalism; the former says that the mind can be reduced down to the physical and is actually identical with our physical brains, while the latter admits that the mind is not reducible to the physical, but has to then explain what the ontology of the mind actually is.  If we choose NonReductive Physicalism then we have to account for mental causation and either say that the mind has no causal efficacy (Epiphenomenalism) or that it does but is not immaterial.[13]   

The Leap Beyond Science

            Once we understand what CC is and where it leads us we must understand it as a philosophy.  What I am pointing at is that CC does not simply come from observing the interactions of physical objects, rather, it is the sort of “philosophical glue” which binds a physical theory together to become a system.  It is not as if we can observe natural phenomena and read off of it CC, but what we do is watch the interactions of physical entities and conclude that causality is strictly physical.  We say that since we have observed past objects to behave in such a way that we can expect the same in the future (Uniformity of Nature/ Induction) and since we have never scientifically observed a non-physical cause to cause anything physical[14] we conclude the Causal Closure of Physics.  We believe that since physics does not operate by any non-physical causation that it cannot ever do so and that the laws of Physics are not merely descriptive but are prescriptive, telling us the way things have to behave. 

            It seems that being frightened by the seemingly illogical position of overdetermination (even though Physicalists have it as well) or the speculative position of Cartesian Dualism will lead one to believe that CC is the only viable alternative, and thus end in Physicalism.  Yet if Physicalists accept the difficulty of overdetermination, then the only trouble they should have is the fear of unscientific Dualism, which is not the location one needs to end at.  In light of this, let us now consider some arguments against CC and see if they lead us to another conclusion.

Arguments Against Causal Closure

          Since CC (specifically Two-way CC) implies that immaterial entities cannot have causal influence in the physical world then if we can provide examples of this happening we can conclude that CC is false.  I will provide three methods of doing this: through Physics (Big bang), Mentality (Causally Efficacious Qualitative States), and Immaterial Laws (Logical and Physical).  I feel that each of these aspects of the universe reveal that non-physical entities do play causally efficacious roles within the physical realm and thus contradict the CC thesis.  First I will examine the non-physical character of the Big Bang, then the nature of Mental States, and finally how Immaterial Laws play causal roles within the physical world.

Big Bang

            The scientific theory of the origin of the Universe, called the Big Bang, says that the matter of the Universe was once compacted into an “infinitely dense” ball of heat[15] that exploded and expanded at the speed of light.  This “infinitely dense” object poses the great philosophical problem of singularity.  How can something be infinite and described in finite terms such as “dense”, “object”, or “ball”?  Similarly how can an infinite object be singular?  Many scientists today believe that this infinitely dense ball proves creation ex nihilo (out of nothing)[16], while others hold to the eternality of matter.  The eternality of matter is refuted by the logical contradiction of an infinite series of events.  An actual infinite series of events is impossible, for how can you have a “series” (succession of finite events) if it is infinite (timeless)?  Likewise John Hospers asks the pertinent question of time: “If an infinite series of events has preceded the present moment, how did we get to the present moment? How could we get to the present moment—where we obviously are now—if the present moment was preceded by an infinite series of events?”[17]  Therefore with these conclusions we cannot accept the idea of the eternality of matter. 

            The other position is to hold to creation ex nihilo, for something infinitely dense turns out to be nothing at all; as William Lane Craig has noted: “There can be no object that possesses infinite density, for if it had any mass at all, it would not be infinitely dense”.[18]  If we accept the ex nihilo hypothesis than we must also conclude that something immaterial (whether God, gods, principles, or laws) had causal efficacy upon the physical world, and thus deny CC.  The philosophy of the Big Bang ultimately comes down to an illogical step, whereby matter spontaneously generated by itself[19], or admits to an entity that is immaterial bringing forth matter into existence.  Innumerable debates have been held over this very topic, and since this paper is not dealing specifically with creation ex nihilo, suffice it to say that if you hold a Big Bang cosmology you must logically deny CC.  Yet even if you do not agree with this there are more determining arguments against CC to which I will now turn.

Efficacious Mental States

            If we can establish two premises about the mind and its relation to the physical world then we can conclude that CC is false.  The first is the non-physical ontology (and thus irreducibility) of the Mind and the second is the causal influence of the Mind upon the physical world.  If we can establish these two claims as factual then we can conclude that something immaterial (mental states) has direct causal efficacy upon the physical world, and therefore prove CC false.  Regarding the immaterial nature of the mind, David Chalmers has shown that the mind is irreducible to the physical through modal arguments of the inverted spectrum or the conceivability of Zombies.[20]  These arguments leave token identity theory as an implausible way of explaining phenomenal consciousness and in fact show that qualia cannot be identical to the physical brain and its processes.

            Similarly the efficacy of the mind upon the physical realm can be proved in various ways.  Firstly “common sense” shows us that our first person experiences contribute to our formulations of thought patters, worldviews, and in turn affect the actions we take.[21]  What it is like for me to experience heat largely contributes to my

expressions of what heat is like, and in turn the affects my actions towards being around heat.  To say that our qualitative experiences have no efficacy upon our behavior (Epiphenomenalism) is to deny not only our common sense ideas of mental influence, but to render the mind obsolete and superfluous in relationship to the physical world.[22]  Thus if we conclude that the mind is immaterial and that it has a causal role in the physical world we are led to deny CC.  This does not mean that physical effects do not have physical causes; rather, it means that effects have more than just physical causes and therefore need to be explained by appealing to more than just physics.

Immaterial Laws

            Physics, as we now have come to understand it operates by laws; laws of heat, energy, light, force, motion, and gravity.  These laws are held to be necessities and those forces by which we can understand the way physical objects behave and interact.  These laws themselves are not physical, they are immaterial.  You cannot touch the 2nd law of thermodynamics, or smell the laws of motion, or see the law of gravity.  Similarly laws of logic operate based upon necessity and are immaterial by nature.  All of these laws, though immaterial, seem to be causally connected to the physical world.  For instance, if the law of identity did not exist then I would not be able to say “this is this” or “that is that”.  Likewise if the law of gravity were not around I would not behave the same way that I do today, I might jump out of windows to get around or try not to jump as high as I can when playing basketball.  These sorts of counterfactuals reveal the effectual nature of immaterial laws to influence physical behavior. 

            One objection to this argument is that immaterial laws are one of many abstract entities, including numbers, properties, values, and propositions, and we would not want to say that the number 2 does anything, therefore abstractions are causally ineffectual.  The difference here is that while 2 does not do anything, they also, unlike logical laws and physical laws, are not necessary.  There is a great difference between a number, value, and proposition, compared to physical laws that govern the physical world and logical laws that govern the mental world.  Another argument posed against this idea is that these laws are merely the “rules” of the physical game that we play and we would not want to say that rules of games have causal efficacy upon the game itself.  Here is where I would make a distinction between interactive causality and counterfactual causality.  It is true that these laws do not physically play in our game, but it is true that, had not the rules been there, our behavior would be quite different, thus showing the default causal influence of laws.

The Scientific Dualist

            We have seen how the Big Bang, Mental States, and Immaterial Laws affect the way we behave, and thus how CC is unwarranted.  The question now is what are we left with?  Do we know throw in the towel on physical laws and their regularities and ability

to explain physical interactions and phenomena?  Of course we do not.  The only difference between the Interaction Dualist and the Physicalist regarding the scientific method is that the former does not take the step to say that we cannot appeal to anything immaterial to explain causal connections.  The Dualist is more frightened by the possibility of the Mind being ineffectual[23] than by the bind of overdetermination.[24]  Therefore when it comes to causal accounts the Dualist does not simply default to something immaterial, but does not shut off the possibility of such, as CC does. 

             The sort of position that I am seeking to defend, which denies CC, is one where the Mind and other immaterial entities play causal roles within the physical world.  The Mind as an immaterial object directly forms our thoughts, aids in making our perceptions intelligible, and categorizes specific data; so there is to be causal room for the Mind to inform our intentions, desires, aspirations, tendencies, and behavior.  In this view the Mind is more than just a superfluous organism or projection; rather it is key in determining physical behavior.  The social sciences have long understood the connection between the Mind and physical behavior and due to the truth of this interaction psychology and sociology find themselves pragmatically verified.  On the contrary CC and its following forms of Physicalism bring physics up as the sine qua non tool for describing human behavior and function and in turn deny the psychological sciences foundational assumption of mental causation.  This is a step I find unnecessary and misguided, for our worldview fits better when we give the Mind an appropriate position within the physical world.


            In conclusion, we have seen how various non-physical entities play causal roles within physics.  Since the Mind is irreducible and immaterial and it has causal impact within the physical world, we must conclude that the CC thesis is unwarranted.  CC does not get read off of raw physical data; rather, an assumption is brought to physics whereby we conclude that physics is causally closed.  This hidden premise in CC must not go unnoticed, for in it the conclusion is proven.  Yet if the premise that nothing non-physical can have causal contributions in the physical world is disproved then we are left denying the conclusion that the world is causally closed.  If the world is not causally closed then we can give the Mind a place within the explanation of behavior and make sense out of qualitative experience.  When we deny CC we in turn deny that the world is only a machine and that humans are only a collection of atoms.  

[1] Lund, David H, The Conscious Self, Humanity Books: New York, 2005, pg 366.

[2] At this point I will use what is known as the “Completeness of Physics” interchangeably with “Causal Closure”, for two reasons: first, most scholars in Philosophy of Mind use these terms synonymously and second, I feel that the “Completeness of Physics” implies Causal Closure because if physical causes fix all physical effects, as Completeness says, then there is no room for non-physical causes to have any efficacy within the physical system, thus closing off the physical realm.  Some scholars say that it does not (Robert Bishop) and some say that Completeness needs an exclusion clause to imply Closure (Jaegwon Kim); I will cover these issues under my section “Completeness vs. Closure”.

[3] Marcus, Eric, Mental Causation in a Physical World, article in Philosophical Studies.

[4] Crane, Tim, The Mental Causation Debate, article from the Proceedings of the Aristotelian Society Supplementary Volume LXIX, 1995, pg 6. 

[5] Bishop, Robert, The Hidden Premise in the Causal Argument for Physicalism, accepted for publication in Analysis, pg 1.

[6] Many scholars make a distinction between Strong Causal Closure and Soft Causal Closure, saying that the former rules out non-physical causes and the latter allows for them.  The reasons for this are that Soft Causal Closure could allow for non-physical causes as long as they are not sufficient causes or as long as we accept overdetermination.  Another way of putting CC is defining it as Two-way Causal Closure which means that “there is no causal interaction between the physical and the non-physical” (Barbara Montero, Varieties of Causal Closure, pg 1).  I feel these debates are semantically driven and under close inspection Soft Causal Closure is used by scholars to refer to Two-way Causal Closure and sometimes Strong Causal Closure.        

[7] Bishop, Robert, The Hidden Premise in the Causal Argument for Physicalism, accepted for publication in Analysis, pg 1, and Barbara Montero, “Varieties of Causal Closure”, section 1.

8 Eric, Mental Causation in a Physical World, article in Philosophical Studies.

9 If we want to hold to true non-physical causality then we must explain how much causal weight is exerted by these non-physical entities.  If we hold this view we must explain overdetermination since we also have the belief that physical causes are sufficient causes.

10  See Peter Menzies “The Causal Efficacy of Mental States”, section 3.  Dualism has to account for overdetermination because there are additional causal influences although physics is held to have sufficient causes, and Physicalism must account for it because there are purely physical instances of overdetermination where multiple causes simultaneously are sufficient to bring about the same effect.

[11] Kim, Jaegwon, Physicalism or Something Near Enough, Princeton University Press: New Jersey, 2005, pgs 149-150.

[12] Some would want to question this point regarding Pantheism saying that they believe that the material world is God and therefore are Physicalists.  The problem with this is that Pantheism admits to another substance, Spirit, which is immaterial and connected to the material world.

[13] Here is the Problem of Mental Causation in a nutshell.  This will be discussed more under “Arguments Against Causal Closure”.

[14] That is, we have never observed it empirically, but there are philosophical arguments for non-physical causation (Big Bang, Mental Causation, The Efficacy of Immaterial Laws, etc.).  I will bring these up when providing arguments against CC.

[15] Another theory is that the this dense ball was not hot but cold, however, this cannot account for the amount of energy contained within the ball itself leading most modern scientists to reject it.

[16] For example: Hugh Ross and Fred Hoyle. 

[17] Hospers, John, An Introduction to Philosophical Analysis, 2nd ed., Routledge & Kegan Paul: London, 1967, pg. 434.

[18] Craig, William Lane, Philosophical and Scientific Pointers to Creation Ex Nihilo, in Journal of the American Scientific Affiliation (1980), 32, pg 5.

[19] This is illogical because it contradicts the logic of causality: something must come from something, nothing comes from nothing, and nothing cannot produce something.

[20] Chalmers, David, The Conscious Mind, Oxford University Press: New York, 1996, under “The Irreducibility of Consciousness”.

[21] Andrew Clifton notes this, in connection the CC theory, when he says “Common-sense experience tells us, however, that our first person thoughts and feelings have definite causal affects upon our physical behavior” (An Empirical Case Against Materialism, 2004, pg 2, at: ).  John Depoe says something similar: “Because physicalism is committed to the causal closure of the physical, at best physicalists can say mental states are epiphenomenal. But it seems obvious that some mental states are causally efficacious, therefore, something is wrong with physicalism.” (Italics Mine, in An Argument Against the Mind Being a Physical Mechanism). 

[22] Paul Raymont comments on Levine’s arguments against Kim’s physicalism: “For example, Joseph Levine takes the problem to be that in nonreductive physicalism every mental property turns out to be causally redundant, since it “adds nothing” to the underling physical state’s causal power to produce the same behaviour” (Kim on Overdetermination, Exclusion and Nonreductive Physicalism, 2002). 

[23] Epiphenomenalism has also lead may Physicalists to hold to “Eliminative” Physicalism, which states that the mind is as unreal as ghosts and fairies (see P.M. Churchland’s “Eliminative Materialism and Propositional Attitudes” in Journal of Philosophy # 78, 1981, pgs 67-90).

[24] This is not to mean that overdetermination is not a problem.  It is a logical problem that should be thought through and the most reasonable conclusion should be held to.