Can Physics Explain Consciousness?

Carl Sagan, the well-known astrophysicist and science popularizer, once declared,

“I am a collection of water, calcium and organic molecules called Carl Sagan. You are a collection of almost identical molecules with a different collective label.” 1

Strange as that assertion may sound, it expresses an idea that many of our contemporaries regard as so obvious as to be beyond dispute.  The idea is called “physicalism” and holds that all of reality, including the human mind, is ultimately nothing but matter in motion and therefore explicable in terms of physics.

This philosophy has ancient roots but in its modern form was inspired by a number of scientific developments, which is why it is sometimes called “scientific materialism.”

One of those developments was Newton’s discovery in the late 17th century of the laws of mechanics and gravity that now bear his name.  Those laws were “universal,” because they applied to all matter throughout the universe, and “deterministic,” because they rigidly and uniquely determined how events would unfold from any given starting point.  A classic statement of “physical determinism” was made by the great mathematician and physicist Pierre-Simon Laplace in 1814:

“[To] an intelligence which could know all the forces by which nature is animated, and the states at some instant of all the objects that compose it, … nothing would be uncertain; and the future …  would be present to its eyes.” 2

All of the fundamental mathematical laws of physics discovered in the two centuries after Newton were also deterministic and this gave rise to the idea that the physical world is a completely closed system of cause and effect, with physical effects being entirely accounted for by prior physical causes. This is often called the “principle of causal closure” of the physical world.

Despite the fact that nature seemed in this way to be in the iron grip of the laws of physics, many scientists and other thinkers continued to believe that Life and Mind must be exceptions and involve the operation of non-physical forces and principles.  But with respect to life this view was undermined by scientific advances in the 19th and 20th centuries that showed that the processes that go on in living things are based on the same chemical laws found in non-living things. And with respect to mind, it was hard to see how it could be an exception either, if the principle of causal closure is assumed to be correct.  Our thoughts (for example, our beliefs) obviously affect our actions and therefore the physical world.  So, if physical effects can have only physical causes, our thoughts must themselves be just physical entities.  As a science writer in Newsweek once baldly put it, “Our thoughts are not mere will-o’-the-wisps, ephemera with no physicality, … they are instead electrical signals.” 3

It should be noted that the discovery of quantum mechanics in the 1920s showed that the laws of physics are not in fact deterministic and do not rigidly determine how events will unfold.  Strangely enough, however, this has not shaken the belief of many philosophers in the principle of causal closure of the physical world, which therefore now seems to be supported more by ingrained intellectual prejudice than by anything science has actually discovered.

The idea that minds are just the operation of physical machines and thoughts are electrical signals started to gain ground with the development of programmable electronic computers in the late 1940s, after which the term “thinking machines” became popular. And now we have reached the point where machines can “outthink” us in activities whose mastery was once considered to require great intelligence, if not genius.  For example, the chess-playing programs Stockfish and AlphaZero are now much stronger than the best human players.

At the same time, rapid advances in neuroscience have demonstrated, sometimes in dramatic detail, the intimate connections between particular physical structures or processes in the human brain and particular mental phenomena.  Many therefore regard it as axiomatic that neuroscience will eventually be able to account for every aspect of our minds.  Francis Crick, co-discoverer of the structure of DNA, once wrote in a popular book,

“‘you,’ your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules. As Lewis Carroll’s Alice might have phrased it: ‘You’re nothing but a pack of neurons.’” 4

One reason that physicalism is an attractive philosophy is that it promises a single, simple, and all-embracing explanation of reality.  It thereby satisfies the natural and proper desire of the human mind for unity and simplicity in its understanding of the world.  This striving for unity and coherence was well-described by Pope St. John Paul II in his 1988 letter to the Director of the Vatican Observatory:

“Unity involves the drive of the human mind towards understanding … . When human beings seek to understand the multiplicities that surround them, when they seek to make sense of experience, they do so by bringing many factors into a common vision. Understanding is achieved when many data are unified by a common structure. The one illuminates the many: it makes sense of the whole. Simple multiplicity is chaos; an insight, a single model, can give that chaos structure and draw it into intelligibility.” 5

This striving for unity of understanding has been rewarded with enormous success in physics. Indeed, the whole trend of theoretical physics over the last four hundred years has been toward an ever more unified conception of physical phenomena.  First, Newton showed that the same laws govern terrestrial and celestial phenomena.  Later, Newtonian mechanics was able to explain phenomena as diverse as the motion of fluids, the propagation of sound, and the flow of heat.  In the 1860s, James Clerk Maxwell showed that electricity, magnetism, and optics are all aspects of a single force, now called “electromagnetism.”  In 1905, Einstein showed that space and time form a unified four-dimensional whole, and that mass and energy are really the same thing.6  In the mid-twentieth century, quantum field theory showed that forces, wave phenomena, and particles are all manifestations of “quantum fields.”  Now physicists are closing in on a single unified theory of all of physics, which may well be so-called “superstring theory.”

It seems very natural, therefore, to imagine that we are headed toward an even grander synthesis in which not only matter, forces, space, and time, but all of reality, including mind itself, will be shown to be explained by a single theory of physics, a “Theory of Everything.”

But there is a danger in pushing any idea too far.  It is possible to desire unity and simplicity of explanation so strongly that one is satisfied to obtain it on the cheap by ignoring or discounting what does not fall within one’s scheme.  Einstein said in a lecture in 1933,

“It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience.” 7

One fatal mistake of physicalism is precisely that is has “surrendered” a very important “datum of experience,” namely experience itself.

Obviously, there can be no “data of experience” unless there are experiences; and there can be no experiences without minds that can have experiences, in other words, consciousness.  The eminent cosmologist Andrei Linde made the following observation in one of his research papers:

“The standard assumption is that consciousness, just like space-time before the invention of general relativity, plays a secondary, subservient role, being just a function of matter and a tool for the description of the truly existing material world. But let us remember that our knowledge of the world begins not with matter but with perceptions. I know for sure that my pain exists, my ‘green’ exists, and my ‘sweet’ exists. I do not need any proof of their existence, because these events are a part of me; everything else is a theory. Later we find out that our perceptions obey some laws, which can be most conveniently formulated if we assume that there is some underlying reality beyond our perceptions. This model of [the] material world obeying laws of physics is so successful that soon we forget about our starting point and say that matter is the only reality, and perceptions are nothing but a useful tool for the description of matter. This assumption is almost as natural (and maybe as false) as our previous assumption that space is only a mathematical tool for the description of matter. We are substituting [for the] reality of our feelings … the successfully working theory of an independently existing material world. And the theory is so successful that we almost never think about its possible limitations.” [Emphasis in the original.] 8

Before proceeding further, it is important to make a distinction in order to avoid a possible misunderstanding.  The issue I am addressing in this article is not whether consciousness is a natural phenomenon, but whether it is a physical phenomenon, i.e. the kind of thing that can be accounted for by physics.  Physicalists claim that it is.  What Linde and many others have suggested is that consciousness, while being a feature of the natural world, just as matter is, may be something just as fundamental as matter, rather than being reducible to it.

Some might object that if matter and consciousness are both features of the natural world then there ought to be some unifying explanation of both, That is certainly a reasonable supposition; but a unifying explanation of two things, A and B, does necessarily have to reduce A to B or B to A.  Consider an example from within physics. We have an enormously successful and well-established theory of electromagnetism. It gives a unified understanding of both electromagnetic fields and electrically charged particles, but it treats them as distinct and equally fundamental realities.  It does not say that electromagnetic fields can be reduced to or are composed of charged particles, nor does it say that electrically charged particles can be reduced to or are composed of electromagnetic fields.  Rather, the theory shows how the two realities are related to each other. There are equations that describes the effect of charged particles on electromagnetic fields (Maxwell’s Equations) and there is an equation that describes the effect of electromagnetic fields on charged particles (the Lorentz Force Law).  In the passage quoted above, Linde makes use of a different example from physics, namely how Einstein’s theory of gravity (General Relativity) treats both material particles and space as equally fundamental realities, without trying to reduce either one to the other.

But even if we were to grant, for the sake of argument, that a unifying explanation of two things, A and B, must reduce one of them to the other.  Which one should be reduced to which?  The philosopher Georges Rey has written,

“[A]ny ultimate explanation of mental phenomena will have to be in non-mental [i.e., material] terms or else it won’t be an explanation of it.”  [Emphasis in the original.] 9

However, one could just as well turn this argument around and say,

“Any ultimate explanation of material phenomena will have to be in non-material [i.e., mental] terms or else it won’t be an explanation of it.”

So there is no a priori reason to expect that all mental phenomena, including consciousness, must be reducible to matter or explained by physics, as the physicalist assumes.

To be sure, there are very good reasons to suppose that consciousness is intimately connected to matter, and that the two together form the single integrated system that we call “nature.”  For example, consciousness only seems to arise when certain kinds of physical structures are present: at least nervous systems and probably brains.  Few people would take seriously the idea that inanimate objects, or plants, or those animals that lack nervous systems, such as sponges, are endowed with consciousness.

Moreover, we know that physical changes in the brain result in changes in conscious experience in ways that are somewhat predictable.  For example, a lower-than-normal concentration in the brain of the molecule dopamine can lead to the subjective experience of boredom or apathy. Damage to a certain small area of the visual cortex called “V4” can strip color from one’s visual experiences.10 The electrical stimulation of a certain tiny region of the brain produces mental states ranging from mild amusement to hilarity. (As reported by the researchers, “at low currents only a smile was present, while at higher currents a robust contagious laughter was induced.” 11)

It seems justified, therefore, to say that physical events “naturally cause” conscious effects, and that matter in certain configurations “naturally causes” consciousness to arise.  The question, however, is whether this “natural causation” is physical causation, i.e. the kind of causation that physics can explain.  There are strong, even compelling, reasons to say that it is not.

To understand this, one must first ask, how does physics explain things?  The answer is that physics explains phenomena by showing how they follow from the basic laws of physics.  In particular, physics explains how a physical cause produces a physical effect by showing that the effect can be derived from the cause using the equations that express the basic laws of physics.  For example, if one places a cup of water in a microwave oven and turns it on for a sufficient period of time, the water will boil.  This can be explained by showing how the oscillating electric field of the microwave radiation pushes around the electrically charged particles in the water molecules and imparts energy to them, how this energy is dissipated as heat due to collisions among the molecules, and how this causes a phase transition of the water.  All of this follows from the equations of electromagnetism and other known laws. That is because both the cause (the microwaves) and the effect (the boiling) are configurations of and changes in matter that are describable by mathematical variables that appear in those equations.

Consider, by contrast, how lowered dopamine levels in the brain cause the feelings of apathy or boredom.  How the dopamine molecules interact with the various other physical constituents of the brain and the consequent effects on brain functioning can certainly be accounted for by physics, in principle if perhaps not yet fully in practice.  But the feelings of boredom and apathy that result are something quite different. Those feelings are not calculable using the equations of physics, like the positions of atoms within a dopamine molecule.  There is no boredom or apathy variable appearing in the laws of physics.  Nor can one measure and thus quantify boredom and apathy in the same way that one measures the positions of atoms.

One can, of course, observe and measure physical effects of boredom and apathy, such as expressions on a face, or the sounds of groans or tapping fingers.  The feelings of apathy and boredom, however, are part of the inner, ‘first-person’ subjective experience of the person having them, just as his (or her) pain, ‘green’ and ‘sweet’ are, to use Linde’s examples.

Similarly, there can be little doubt that physics and chemistry can explain how the cone cells in the retina react to electromagnetic waves of different wavelengths and encode that information in signals that are sent up the optic nerves, and how the cells in the V4 region of the visual cortex allow the brain to interpret this information and distinguish the different wavelengths of light.  But the private, subjective, incommunicable experience of ‘green’ is something altogether different.

The philosopher Frank Cameron Jackson12 invented a famous argument concerning such sensory experiences (which philosophers call “qualia”). He considered a hypothetical person (whom he named “Mary”) who knows everything there is to know about the laws of physics and about human biology, including the physiology of human color perception, but who was raised from birth in a completely black-and-white environment and had therefore never experienced color sensations.  If she then saw the color red for the first time, would she have learned something about reality she had not known before?  Obviously, yes.  But that “something” isn’t something she could have derived from her previous complete knowledge of physics and human physiology.  There is something about the world of experience, therefore, that eludes the grasp of the equations of physics.

One can make a broader argument along the same lines.  Even a complete description of any physical system or entity, down to the configurations and changes of its most basic constituents (whether they are elementary particles, such as electrons and quarks, or are superstrings or something else) and a complete knowledge of the equations of physics would not be enough to reach any conclusion by mathematical and logical deduction about whether that system or entity was experiencing anything, i.e. whether it had consciousness.  For example, it might be foolish and perverse of me to deny that my dog has consciousness and subjective experiences, but if I did deny this I would not be contradicting anything we know about physics and chemistry.  Nor would I be involved in any logical contradiction.  A normally functioning dog that is running around and barking but is utterly devoid of consciousness and subjective experience may be contrary to the order of nature, i.e. to the way the universe actually works, but it cannot be shown to contradict the equations of physics.

Arguments along these lines have been made by several eminent contemporary philosophers, including Saul Kripke, Thomas Nagel,13 and David Chalmers.14  They point out that one can conceive of a universe that had the same laws of physics as ours and that contained living beings that were physically constituted just like human beings or other animals in our universe and yet utterly lacked consciousness.15  There seems to be nothing logically or metaphysically impossible in the idea of such a universe.  In our own universe, it does seem to be the case that beings that are physically constituted in certain ways have consciousness, but this cannot be shown to follow by logical necessity from the laws of physics.

It is not only some eminent philosophers who have made this point, but some eminent physicists as well.  Some years ago, the theoretical physicist Edward Witten, regarded by many of his colleagues as the most brilliant physicist of his generation, was asked by an interviewer whether he thought humans would ever know the ultimate laws of physics and whether, if that day came, anything would still remain “mysterious.”  Witten responded that he thought several things would likely remain mysterious, including consciousness.  These were his words:

“I tend to think consciousness will be a mystery. That’s what I tend to believe. I tend to think that the workings of the conscious brain will be elucidated to a large extent … , that biologists, [with] perhaps physicists contributing, will understand much better how the brain works. But why something that we call consciousness goes with those workings I think will remain mysterious. Perhaps I’m mistaken, [but] I have a much easier time imagining how we’d understand the Big Bang, even though we can’t do it yet, than I can imagine understanding consciousness.” 16

When pressed by the interviewer, who was evidently not pleased by this answer, to “define” consciousness, Witten responded,

“I am not going to attempt to define consciousness. In a way that is connected to the fact that I don’t think it will become part of physics. … I am skeptical that it’s going to become part of physics.”

Erwin Schrödinger, one of the founders of quantum mechanics, wrote,

“[While] all scientific knowledge is based on sense perceptions, the scientific view of natural processes formed in this way lacks all sensual qualities and therefore cannot account for the latter.” 17

In an interview, he also said,

“Consciousness cannot be accounted for in physical terms.” 18

At this point, a question naturally arises.  If physics is not a complete account of nature, as the physicalist believes it is, shouldn’t there be some hints of that incompleteness coming from physics itself?  If something is incomplete, shouldn’t there be gaps or loose ends or ragged edges in it pointing to its incompleteness?  And yet, as physics has progressed, its theories appear ever more tightly integrated and “rounded off,” so to speak.  It appears more and more that the ultimate fabric of physical theory will be “seamless, woven in one piece from top to bottom,” if one may borrow a scriptural image.

There is, however, one strong indication from physics itself of its own incompleteness.  It was pointed out in 1930 by John von Neumann in his classic book The Mathematical Foundations of Quantum Mechanics.19  Quantum mechanics, which is the fundamental grammar of all physics, posits two kinds of entities: “systems,” which are observed and measured, and “observers,” who make the observations and measurements.  The equations of physics describe systems but not observers, at least not fully.  If one tries to stretch the equations to describe the whole process of measurement, including both the system and the observer in their totality, one runs into a fundamental problem.

The problem is that the equations of quantum mechanics only yield probabilities of various possible outcomes of measurements, whereas a measurement, to be such, must yield a definite outcome.  Therefore, if everything is described by the equations, then only probabilities can result from the calculations rather than any definite outcome, meaning that no measurement ever happens.  This is the classic “measurement problem.”

Some eminent physicists have argued that the only way out of this conundrum is to say that something about the observer, in particular his or her knowledge and consciousness, is not reducible to physics and cannot be included in its mathematical descriptions of the world.  For example, Sir Rudolf Peierls, a leading twentieth century physicist, said,

“[T]he premise that you can describe in terms of physics the whole function of a human being … including its knowledge, and its consciousness, is untenable. There is still something missing.” 20

In a famous essay on the measurement problem, the physics Nobel laureate Eugene P. Wigner wrote that “materialism” is not “consistent with present quantum mechanics.” 21 Commenting on this, he said,

“It will remain remarkable, in whatever way our future concepts develop, that the very study of the external world led to the conclusion that the content of the consciousness is an ultimate reality.” 22

These conclusions about the implications of quantum mechanics are controversial and most physicists would be very reluctant to accept them.  Nevertheless, it does seem that fundamental physics is pointing to its own radical incompleteness.

So far, the discussion has been about physics and philosophy, but some readers may suspect that lurking behind this discussion is religion. They would be partly right, but several points should be made in this regard.

First, many of the philosophers and physicists who have doubted or denied the sufficiency of physics to account for consciousness are not religious. That is the case of almost all of those whom I have quoted above, including the physicists Erwin Schrödinger, Rudolf Peierls, Eugene Wigner, Andrei Linde, and Edward Witten, and the philosophers Frank Cameron Jackson, Thomas Nagel, and David Chalmers.

Second, biblical religion, and Catholicism specifically, do not teach anything about consciousness and whether it is reducible to physics.  It would be perfectly orthodox for a Catholic to say that lower animals, despite possessing consciousness, can be entirely understood in material terms.  It is not consciousness that is of doctrinal significance to the Catholic Church and Christian tradition, but rather the higher and distinctively human powers of rationality and free will, which the Church (in agreement with a long philosophical tradition) claims to be “not reducible to the merely material.” 23

So nothing is directly at stake for Catholic doctrine in the question of consciousness.  And yet, it does have relevance to religion in an indirect and negative way, for the irreducibility of consciousness to physics is enough by itself to refute physicalism.  And physicalism is perhaps the greatest intellectual obstacle to religious belief today.  By its sweeping claims, physicalism rules religion completely out of court.  If nothing but physical stuff existed, then obviously God and the human spiritual soul could not exist, nor could many of the other realities that Catholic doctrine and Christian tradition speak about.

Most of those today who claim that “being scientific” requires one to be an atheist hold that view because they think that science somehow points to the truth of physicalism.  And yet nothing is more unscientific than physicalism.  For science is based on empirical facts, and there is no empirical fact more basic or more evident than that we are conscious, a fact that physical science, by its very way of proceeding, is powerless to explain.


1.. Carl Sagan, Cosmos (Random House, 1980), p. 134.

2.. Pierre Simon Laplace, A Philosophical Essay on Probabilities (New York: Dover, 1951) p. 4-5.

3.. Sharon Begley, “Thinking Will Make It So,” Newsweek, April 5, 1999, p. 64.

4.. Francis Crick, The Astonishing Hypothesis: The Scientific Search for the Soul (New York: Charles Scribner’s Sons, 1994), p. 3.

5.. ii/en/letters/1988/documents/hf_jp-ii_let_19880601_padre-coyne.html

6.. That is the meaning of the celebrated equation E = mc2, which in  natural space-time units where c = 1 is just E = m.  One way to define the mass of a system is as its energy in its center-of-momentum frame.

7.. Albert Einstein, “On the Method of Theoretical Physics,” the Herbert Spencer Lecture, Oxford, June 10, 1933.

8.. Andrei D. Linde, “Inflation, Quantum Cosmology, and the Anthropic Principle,” p. 26-7.  e-Print: hep-th/0211048 [hep-th]

9.. Georges Rey, Contemporary Philosophy of Mind (Oxford: Blackwell, 1997), p. 21.

10.. Malcolm Jeeves and Warren S. Brown, Neuroscience, Psychology and Religion: Illusions, Delusions and Realities about Human Nature (West Conshohocken, PA: Templeton Foundation Press, 1997), p. 65.

11.. Malcolm W. Browne, “Who Needs Jokes?: Human brain has a ticklish spot,” in The New York Times, March 10, 1998, sec. F, p. 1.

12.. Frank Jackson, “Epiphenomenal Qualia,” The Philosophical Quarterly, 32 (127) 1982, p. 130.  Interestingly, and perhaps tellingly, Jackson later repudiated his own argument, despite apparently having found no answer to it.  He wrote, “Most contemporary philosophers given a choice between going with science and going with intuitions, go with science. Although I once dissented from the majority, I have capitulated and now see the interesting issue as being where the arguments from the intuitions against physicalism—the arguments that seem so compelling — go wrong.”  This, of course, begs the question whether “going with science” requires accepting physicalism.  In any case, “going with science” is clearly not the same as going with “the majority” of “contemporary philosophers.”

13.. Thomas Nagel, Mind and Cosmos (Oxford: Oxford University Press, 2012).

14.. David Chalmers, The Conscious Mind: In Search of a Fundamental Theory (Oxford: Oxford University Press, 1997).

15.. In the colorful parlance of contemporary philosophy, hypothetical entities that are by physical constitution just like ordinary living human beings but devoid of consciousness are called “philosophical zombies.” No one believes that such beings actually exist, of course.  For discussion of the concept and the related arguments against physicalism, see and

16.. Edward Witten, Video Interview with Wim Kayzer, “Episode 9: Of Beauty and Consolation,” July 21, 2014,

17.. Erwin Schrödinger, Mind and Matter (Cambridge: Cambridge University Press, 1958), p. 103.

18.. Interview in The Observer, Jan 11, 1931.

19.. John von Neumann, Mathematical Foundations of Quantum Mechanics, English translation (Princeton, NJ: Princeton University Press, 1955)

20.. Rudolf Peierls, quoted in P.C.W. Davies and J.R. Brown, The Ghost in the Atom (Cambridge: Cambridge University Press, 1986), p. 75.

21.. Eugene P. Wigner, “Remarks on the Mind-Body Question,” in The Scientist Speculates, ed. I.J. Good (London: William Heinemann Co., Ltd., 1961), reprinted in Eugene P. Wigner, Symmetries and Reflections: Scientific Essays (Woodbridge, CT: Oxbow Press, 1979), p. 176.

22.. Eugene P. Wigner, Symmetries and Reflections, p. 171.

23.. Catechism of the Catholic Church, par. 33; Gaudium et spes, Par. 18.  What makes the human soul irreducible to the merely material, is that it is a spiritual soul, which unlike the souls of animals is endowed with the spiritual powers of rationality and free will.  See, for example, Catechism of the Catholic Church, par. 1704-6, 1711.


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