A childish question (literally) -

My 8 year old asked me this morning: "Dad, what does 'physical' mean?" - and I found myself at loss for an ordinary language answer.

Every answer I could come up with ended up sounding like a definition of empiricism: "physical things are those things that can be observed", "physical means something that can be measured", "whatever can be verified and tested", etc....

Claiming that things that are physical are those things which are material or things that are described by physics seems circular.


phys·i·cal ˈfizik(ə)l/ adjective 1. relating to the body as opposed to the mind. "a whole range of physical and mental challenges" synonyms: bodily, corporeal, corporal, somatic; More 2. relating to things perceived through the senses as opposed to the mind; tangible or concrete. "pleasant physical environments" synonyms: material, concrete, tangible, palpable, solid, substantial, real, actual, visible "the physical universe"

And various dictionaries (here Merriam-Webster):

Definition of physical 1 a : of or relating to natural science b (1) : of or relating to physics (2) : characterized or produced by the forces and operations of physics 2 a : having material existence : perceptible especially through the senses and subject to the laws of nature everything physical is measurable by weight, motion, and resistance — Thomas De Quincey b : of or relating to material things

lead to similar definitions.

How can one define "physical" in ordinary language terms? Is there a way of defining "physical" that doesn't presuppose empiricism? What is the rationalist definition of 'physical'?

Is there a definition that doesn't commit to either dualism or empiricism or to any epistemology?

  • Very interesting question. I feel like any sort of proposed, at least philosophical, definition of physical that ignores epistemic commitments or answers to substance duality or monism will probably immediately be confronted with those exact questions. We have done philosophy for so long that we know the question of dualism is at least nominally important, I can't see a description of physical that doesn't commit to a dualism or monism as being meaningful because there are so many unanswered questions about the actual substance. It seems like it will just lead back to the same place.
    – Not_Here
    Commented Apr 3, 2017 at 17:48
  • "Physical" is whatever we can iteract with: smell, touch, see (also with a telescope or with a ionization chamber), measure. Commented Apr 3, 2017 at 18:15
  • 2
    @MauroALLEGRANZA aren't you just rehashing the empiricist definition? Commented Apr 3, 2017 at 18:27
  • As an opinion: I find it effective to define "physical" to mean its behavior can be completely specified by rules. This lines up well with the empirical approach and has some really nifty side effects regarding what it means to blindly follow all rules without question. It also plays nicely when you start exploring the question of "what do I do if I don't know all the rules," which is a pretty reasonable place for any philosopher to start! No citations of philosophers for that phrasing though, so it'd make a poor answer.
    – Cort Ammon
    Commented Apr 4, 2017 at 20:16
  • @CortAmmon: Doesn't that commit to determinism though? A random outcome doesn't individually follow a rule, even if averages do.
    – CriglCragl
    Commented May 7, 2023 at 21:47

5 Answers 5


This question seems to be a companion to How can something non-physical exist? Some preliminary thoughts: acknowledging the existence of empirical, or even confining physical to empirical, does not presuppose empiricism. Even Parmenides and Plato acknowledged the "sensible", as they called it, even if only as "illusion" or "imitation". Later rationalists, Descartes, Kant, Hegel, etc., were even more generous. Moreover, most philosophers, rationalist or not, were keenly aware that to remain credible they ought to validate the ontology and terminology of common sense, and later science, at some level. Kant, for instance, reinterpreted "matter" as the medium of our sensibility, and Newtonian physics as based on categories and synthetic a priori intuitions, but he was careful to make sure that when it came to ordinary use physics remained physics.

The word φύσις meant "nature" in Greek (with the more archaic connotation of "growth"), so the original meaning of "physical" is natural, belonging to nature. Of course, philosophers, in this case especially Aristotle, produced many elaborations of the concept and the current ubiquity of "physical" has its roots in the title of Aristotle's well-known book, Physics. Aristotle relates the characterization to his four causes, and stresses the difference between the natural and the artificial relative to efficient causes, only the natural has its source of motion within itself. Here is Wang's summary:

"We know Aristotle divides sciences (episteme) into three parts: physical science (theoretike), productive science (poietike), and practical science (praktike). (Metaphysics, 1025b 19–25) They concern themselves with different classes of things: physis, techne, and ethos. Physis is of those things that are generated by nature. That from which they are generated is matter. That which they become is form. So physis is characterized such that the form is generated from the matter itself (1032a 16–18). Techne is those things that are generated artificially. In contrast to physis, the form of techne is not generated from matter itself, but from the soul of a human being."

The second major event in the genesis of the modern "physical" occurred in 13th century, and had to do with the reformation of the Aristotelian concept of matter. From signifying relative "potentiality" of any sort whatsoever it came to mean something much more specific, and familiar. Here is Pasnau's Form and Matter:

"For an Aristotelian, the fundamental constituents of physical bodies are not integral parts, but the metaphysical parts of form and matter. On one understanding of matter, it is the counterpart of form – the stuff that gets informed – so that whenever there is a form there must also be some matter that serves as its subject... Wood, for example, is a form–matter composite that can itself serve as the matter of a bed.

[...] Many early scholastic authors, especially Franciscans, embraced this sort of view. From the time of Albert the Great and Thomas Aquinas, however, the view fell entirely out of favor, replaced by the idea that matter occurs only in the corporeal realm. Hence there arose the linkage we take for granted today, between corporeality and materiality, so that to be a body (corpus) is just to have matter."

Moreover, Aquinas explicitly links corporeality to spatial location and motion:

"To receive, to be subjected, and other such things do not apply to the soul and to prime matter in the same way (rationem), because prime matter receives a thing through transformation and motion. And because all transformation and motion goes back to local motion as to what is first and most common (as is proved in Physics VIII), we get the result that matter is found only in those cases where there is the potentiality for location. But only corporeal things, which are circumscribed to some place, are of this sort."

The more familiar characterizations of the physical/natural in Descartes and Spinoza are in terms of extension (res extensa, "the essence of matter is extension", extension as one of two attributes of God/Nature) stem from these scholastic sources.

Of course, since Galileo and Descartes the more modern scientific view of "nature" and "physical" also took hold. The "natural" or "physical" is that which is in accordance with the laws of nature (naturalism), specifically with the causal laws (Aristotle's four causes being reduced to efficient causes only), which ideally are reducible to laws of physics (physicalism), and originally to "particles in motion" (mechanism). This is the notion that features in modern philosophical debates as well, e.g. on the reconciliation of spontaneous subject with the causal world started by McDowell's Mind and World. It is only ostensibly circular, one does not need the notion of physical to establish empirical regularities, so is free to define "physical" in terms of them. In contrast, the "unphysical" is that which does not accord with those laws, and, more recently, something ghostly, unobservabvle in principle, like the luminiferous ether.

Hence, the modern medley of folk intuitions about the "physical": external to the mind/sensible/the outside world (nature), material as corporeal/tangible and spatially extended, and behaving according to (perhaps yet undiscovered) causal laws. For an 8 year old I'd say "physical is what surrounds you, what you can feel, touch and see move and change, scientists study it so they can predict how it moves and changes".

  • It is circular to define physical as what surrounds you. what you can feel and touch..." etc. Yes, this is true, but what is it? We cannot answer directly as we do not know what matter is. Indeed, it appears not to be there when we look, so the word' physical' seems to be a psychological term.
    – user20253
    Commented Apr 4, 2017 at 12:31
  • 2
    @PeterJ Ordinary language users do not know presumably what the matter "is" but it does not stop them from using "physical", nor does it stop philosophers from philosophizing about it. The question is what "physical" means to them. But why is it circular? Definition is circular if the term on the left also appears on the right, but surrounds, feel, touch, etc., do not use "physical".
    – Conifold
    Commented Apr 4, 2017 at 19:36
  • DesCartes makes the most sense in this context. I should have used a simplified version of his divisible/indivisible dichotomy. Commented Apr 7, 2017 at 4:44
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    @AlexanderSKing It originates long before Descartes, in Plato's Phaedo. Socrates argues that the soul is simple, indivisible, has no parts, and consequently cannot decompose, unlike material things. The "simplicity of the soul" and simplicity-to-immateriality arguments were a mainstay in scholastics, from whom Descartes inherited it. Spinoza rejected it. It fell out of favor after Kant pointed out that it confuses functionality with substance, and that materialists can equally claim that the soul is material because we do not know how an immaterial substance can implement "simplicity" either.
    – Conifold
    Commented Apr 7, 2017 at 22:06
  • 1
    @DukeZhou I doubt it. Aquinas was an Aistotelian and the whole passage sounds Aristotelian. He even explicitly refers to Aristotle's Physics. Aquinas, and scholastics in general, were kind of on the opposite end of the spectrum from atomists, who in particular had no use for Aristotle's form.
    – Conifold
    Commented Apr 7, 2017 at 22:45

Take a look at the SEP article on physicalism by Daniel Stoljar: https://plato.stanford.edu/entries/physicalism/

The upshot of that article is that trying to clarify what physical means is a rabbit hole.

Chomsky repeatedly argues that there is no mind-body problem because we do not know what physical means. Here is a snippet from a lecture titled The machine, the ghost, and the limits of understanding: https://www.youtube.com/watch?v=D5in5EdjhD0&t=46m22s

mind body is meaningless. If there's no body, there's no mind-body problem, and there hasn't been any concept of bodies since Newton. I mean, Newton still thought that there was one, but as he put it "we haven't yet discovered it", meaning, accounted for it in mechanical terms, but that's been given up certainly by the 20th century. So there is no concept of physical. The term physical is just kind of like an honorific word.

(The entire lecture is very interesting.)

And here is a paragraph from The Mysteries of Nature: How Deeply Hidden? where Chomsky comments on Stoljar:

The second option is pursued by Daniel Stoljar, who has done some of the most careful work on physicalism and variants of the “mind-body problem.” He does offer some answers to the question of what it means to say that something is physical — a question which, he notes, has not received a great deal of attention in the literature, though “Without any understanding of what the physical is, we can have no serious understanding of what physicalism is.” The answers he offers are not too convincing, I think he would agree, but he argues that it does not matter much: “we have many concepts that we understand without knowing how to analyze,” and “the concept of the physical is one of the central concepts of human thought.” The latter comment is correct, but only with regard to the common-sense concept of the mechanical philosophy, long ago undermined. The former is correct too, but it is not clear that we want to found a serious philosophical position on a concept that we think we understand intuitively but cannot analyze, particularly when a long history reveals that such common-sense understanding can often not withstand serious inquiry.

That said, Conifold's final remark that physical is what we can "see move and change" reminds me of Buddhism and in particular the Mahamudra tradition. According to it the true nature of mind and existence is transcendent, in that it can only be partially described. That which cannot be described or identified (having properties that can be used as criteria for identifying it) is called emptiness, and that which can be described to an extent, is called phenomena, and includes "external" phenomena, sensations, and even thoughts and emotions. Here are a few quotes from The Royal Seal of Mahamudra:

Self-knowing awareness is unidentifiable; that is emptiness. Thoughts and the manifestation of phenomena are the radiance of emptiness. (p.42)

Just like waves rise from the water itself, all phenomena are merely manifestations of the natural radiance of essential mind. (p.279)

The myriad things arising from the mind Appear to people as external. They are just mind, the mind of worldly people. There are no external phenomena— It is the mind that arises as the myriad things. (p.280)

And if you are inclined to dismiss this as mambo jambo, then consider this quote by Niels Bohr:

There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.

According to Buddhism the describable and the indescribable are mysteriously one (non-dual), and therefore all conceptions or descriptions of the physical are inevitably partial, approximate or paradoxical.

Instead of trying to show that the world is intelligible to us, we recognized that it’s not intelligible to us. But we just say, ‘Well, you know, unfortunately that’s the way it works. I can’t understand it but that’s the way it works.’ And then the aim of science is reduced from trying to show that the world is intelligible to us, which it is not, to trying to show that there are theories of the world which are intelligible to us. That’s what science is: It’s the study of intelligible theories which give an explanation of some aspect of reality. (Chomsky)

And as put by Feynman:

[Newton's law of gravitation] is not exact; Einstein had to modify it, and we know it is not quite right yet, because we have still to put the quantum theory in. That is the same with all our other laws – they are not exact. There is always an edge of mystery, always a place where we have some fiddling around to do yet. This may or may not be a property of Nature, but it certainly is common to all the laws as we know them today. It may be only a lack of knowledge.

  • "According to Buddhism the describable and the indescribable are mysteriously one (non-dual)," This sounds like Russell's neutral monism. And this "Just like waves rise from the water itself, all phenomena are merely manifestations of the natural radiance of essential mind." like Berkeley's idealism. so both are ultimately empiricist. Commented Apr 7, 2017 at 4:33
  • From your answer, I think Chomsky is the most relevant - there's a limit to what we can know and we have to live with it. It's almost Wittgensteinian: Something can't be expressed using language, yet they are fundamental. Commented Apr 7, 2017 at 4:38
  • I would be surprised if it was Russellian. Did Russell believe nature transcends logic and reason? in "are mysteriously one" mysteriously is used in the sense given by Chomsky — of being hopelessly beyond our capacity to comprehend. Maybe a more fitting title would be Transcendent Monism.
    – nir
    Commented Apr 7, 2017 at 10:11
  • Yeah - you're right. I doubt Russell would have been a mysterian. Commented Apr 7, 2017 at 21:51
  • @nir thoughts on hypotheses regarding the interchangeability of matter and information? (Props for bringing in Buddhist concepts, not least for linking them to Feynman!)
    – DukeZhou
    Commented Apr 7, 2017 at 22:33

Newtons answer to the question of physicality is imponderability; that is A has a relation of physicality to B if A cannot pass through B (notably, like his third law, there is a principle of symmetry embedded in it).

He speculated that matter when it first appeared was not ponderable, and then ponderability was somehow switched on, so from the general aether one got durable, permanent and ponderable atoms.

The question he asked himself that led to this speculation was what was ponderability, how did the interior of an atom differ from its exterior, and he speculated that the source of ponderability was a force.

This I think was in one of his unpublished manuscripts, at least in his lifetime, presumably due to its speculative nature. Unfortunately I didn't make a note of which one, so I can't quote it - memory will have to serve.


I may have missed it, but it seems nobody has given an answer - at least, nobody has come close to countering Chomsky's view.

I have a definition - well, not really a definition, since I don't think "physical" in the philosophic sense has any valid meaning. I think it's a statement of faith: "I have no idea what physical means, but one thing I know for sure - mind, consciousness, awareness, intelligence, sentience are all derivative of something which is non mental, unconscious, not awareness, unintelligent and insentient. I don't care if there's, by definition, no possibility of even the slightest bit of scientific or empirical evidence for such a purely physical 'thing,' i just believe it with all my heart.

Sam Harris gave this answer in a dialog with Rupert Spira. Sam agreed that all we know is through the medium of consciousness. He said he just "believed" that something "physical" existed before there was consciousness [here, conflating 'consciousness' with human consciousness] without feeling any need to even provide a reason, much less evidence for this.

So ultimately, I can't see any reason why physicalism should not be considered a religion, rather than a philosophy, one based on pure faith.

  • Just because there was 1 idiot, doesn't make physicalism a religion.
    – CriglCragl
    Commented May 7, 2023 at 21:22
  • Every sufficiently advanced philosophy is indistinguishable from mystic.
    – Scott Rowe
    Commented May 8, 2023 at 9:50
  • Welcome to SE. We all believe in things we cannot define and/or things we cannot prove. But in philosophy, that's a problem, not an answer. It helps, when you are quoting something on this site, to say where you found it so that we can all look it up. By the way, there's more to a religion that believing things on faith - a set of values and a way of life, for example. It might help you if you read the help pages on how to answer a question on this site.
    – Ludwig V
    Commented May 9, 2023 at 19:33

"In this sort of predicament, always ask yourself: How did we learn the meaning of this word ("good", for instance)? From what sort of examples? In what language-games? Then it will be easier for you to see that the word must have a family of meanings." -Wittgenstein, in Philosophical Investigations

I find etymology is generally more interesting than a dictionary definition.

from Medieval Latin physicalis "of nature, natural," from Latin physica "study of nature"

The meaning "pertaining to matter, of or pertaining to what is perceived by the senses" is from 1590s; the meaning "having to do with the body, corporeal, pertaining to the material part or structure of an organized being" (as opposed to mental or moral) is attested from 1780.

-from Etymonline

So you can compare physical and unphysical, to natural and unnatural. The appeal-to-nature fallacy illustrates how the ever-shifting hodgepodge of meanings for a superficially simple word, can easily lead to it's use for sophistry.

I see 'physical' as a contrastive, used in language games in sets of contradistinctions (this answer discusses this kind of thing in more detail Life and Death as one and the same?).

So the first pair is physical vs mental, very important for Descartes who made important early steps in defining science.

The other pair is physical vs metaphysical, which owes a lot as a distinction to Aristotle's Metaphysics, which teased apart 'first philosophy' from observational study, but through Scholastic religious philosophy metaphysics also became a kind of dirty word for scientists. Eliminating action-at-a-distance, was a key concern for the developing scientific picture of physicalism, removing the 'mysterious ways' of divine intervention and miracles.

I would say there is also physically real, used in contrast to fictional, or imagined.

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