What is the definition of physical? Is that definition clear enough to make a distinction between physical and non-physical?

Question

Awkwardly synthesizing jobermark's old question Is there a boundary on 'physical'? with my (badly put) question Can physics talk about non-physical entities/concepts, and if not which academic department does?, finally pinpointing (I hope) what bugs me.

Let me start with what I'm hoping to find:

1. Non-physical having causal effects on the physical (and vice versa).

2. Clear enough definition (no matter how general it is) of physicality that'll distinct between physical and non-physical, or, will provide boundaries of physicality.

3. A definition of physicality that'll account for the history of physics, the changes on how we viewed physics and non-physics.

4. Not a dualistic solution.

In jobermark's question he presents the idea that the definition of physicality is so vague that it always expands and encompass what was in the past not considered physical (similar, in general, to the vagueness of "the scientific method"). Then he goes to state that this makes the distinction between physical and non-physical as near non-existent nowadays*.

My question was about the academic scope of this question, as in, not only conceptually, but practically - is a physicist able to talk about the non-physicality. This question seems more clear to me after going through jobermark's because it means that the physicist, if he talks about non-physical entities, might not know that it's in fact what he does because to him it seems like physical entities (because of, again, the vagueness of the definitions).

So after pinpointing that the root of the issue was at the definition level (which I basically tried to ignore in my question), and where jobermark's answers weren't suffice (at the end I think he took a turn in his answer, which seems to me like a bit of avoiding the problem) because while admitting the problem they haven't really provided a concrete answer (maybe there isn't one, and if so this question can be deleted).

So, my question will be - is there any clear enough definition of physics that answer the 4 requirements I've stated above?

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(*although there are the ways that's been shown in the answers to Alexander's question How can something non-physical exist?, the first positing the distinction in abstractness/concreteness but agrees to providing physicality all of the causal realm [which isn't what I'm seeking]; the second doesn't really distinct them but rather takes a sort of holistic/monistic approach, which isn't bad but doesn't yet answers the question to its full extent [even in this holistic approach one needs to distinct where the physical end and the non-physical starts]; and another answer distributes the physical and non-physical to different areas of existence, which [I think] renders every attempt to bring non-physical causality to science as futile)

Answer 1

The best definition is that which Kant applies in his works: physical (empirical) knowledge corresponds to that which originate from the senses, and metaphysical (rational) is what does not originate from the senses, that is, from reason.

Such approach originates in earlier philosophical traditions, particularly the works of empiricists like Locke and rationalists like Descartes. However, Kant's contribution was synthesizing this idea as a core concept in his critical philosophy, particularly in his "Critique of Pure Reason,".

You will notice that this definition is quite arbitrary; here is a simple approach of it: for example, aesthetics is considered part of metaphysics, but it depends on physical knowledge (listening music depends on hearing, appreciating a sculpture depends on sight, touch, etc.). So, this perspective would finally imply that all knowledge is physical.

Kant focuses such problem and asks himself: given that most knowledge depends on experience [the senses] (the heritage from earlier Empiricists), is there some knowledge that is purely rational [metaphysical] (what Rationalists believed, but without a solid foundation)?

So, he identifies what he calls pure knowledge, (implying pure metaphysical knowledge), knowledge that is not a direct knowledge coming from experience (the senses), but knowledge that is created a priori (necessary) to have experience. The knowledge that Kant includes in this category includes few abstract intuitions, like space, time, and the 12 categories.

The term pure gives name to Kant's masterpiece, Critique Of PURE Reason.

For your goals, physical is all phenomena that is directly perceived by means of the senses. All knowledge that comes not directly from the senses is metaphysical, whether it is pure or not.

Answer 2

Its a conceptual distinction to understand our experience. Theres only experience, but we categorize it to understand it. Einstein said:

"A basic conceptual distinction, which is a necessary prerequisite of scientific and prescientific thinking, is the distinction between “sense-impressions” (and the recollection of such) on the one hand and mere ideas on the other. There is no such thing as a conceptual definition of this distinction (aside from circular definitions, i.e., of such as make a hidden use of the object to be defined). Nor can it be maintained that at the base of this distinction there is a type of evidence, such as underlies, for example, the distinction between red and blue. Yet, one needs this distinction in order to be able to overcome solipsism. Solution: we shall make use of this distinction unconcerned with the reproach that, in doing so, we are guilty of the metaphysical “original sin.” We regard the distinction as a category which we use in order that we might the better find our way in the world of immediate sensations. The “sense” and the justification of this distinction lies simply in this achievement. But this is only a first step. We represent the sense-impressions as conditioned by an “objective” and by a “subjective” factor. For this conceptual distinction there also is no logical-philosophical justification. But if we reject it, we cannot escape solipsism. It is also the presupposition of every kind of physical thinking. Here too, the only justification lies in its usefulness. We are here concerned with “categories” or schemes of thought, the selection of which is, in principle, entirely open to us and whose qualification can only be judged by the degree to which its use contributes to making the totality of the contents of consciousness “intelligible.” The above mentioned “objective factor” is the totality of such concepts and conceptual relations as are thought of as independent of experience, viz., of perceptions. So long as we move within the thus programmatically fixed sphere of thought we are thinking physically. Insofar as physical thinking justifies itself, in the more than once indicated sense, by its ability to grasp experiences intellectually, we regard it as “knowledge of the real.” After what has been said, the “real” in physics is to be taken as a type of program, to which we are, however, not forced to cling a priori." A. Einstein, “Remarks concerning the essays brought together in this co-operative volume,” in Albert Einstein: Philosopher-Scientist, P. A. Schilpp, ed., pp. 665–88. Tudor Publishing Co., New York, 1949

For me we can talk about interior experiences, and external (physical) ones. The internal world acts on the external one all the time.

Answer 3

Aristotle invented the word τὰ ϕυσικά ("ta physika," lit. "natural things"), which is the collective title of his physical treatises. Natura comes from natio = birth, so natural [or physical] things are generated, the product of a change.

^(source)

Answer 4

As a partial list, here are a few different things, which may or may not exist, and which may or may not consist of the same fundamental things:

1. The place where all the things are that we can experience (directly or indirectly) with our basic senses - see, hear, smell, taste, touch.

   Note: I'm specifically not saying simply "the things we can experience with our senses". Because most people would agree e.g. that some particular distinct star is physical, even if it may be impossible, given the laws of physics, for us to experience that particular star with any of our senses.

2. Consciousness

3. Gods, angels, demons and the like

4. The afterlife

5. Ghosts

6. Various "mystical" forces such as reincarnation, moving stuff with your mind, the "memory" of water proposed by pseudoscience, etc.

"The physical" is typically broadly-speaking agreed to be point #1 (at least) - what we experience with our senses. There may be some confusion / contention here in terms of any of the other items above being indirectly observable - more on that below.

(Reductive) physicalists would say that #2 - consciousness, reduces* to the stuff that's going on in your brain, and it's therefore also physical.

* A new consciousness pops up as a result of creatures rubbing their private parts together. We've identified which part of the brain does what. Brain scans show stuff happening in the brain corresponding to what people think. Injury, mental conditions and diseases directly affect our consciousness (in particular: damage to a part of one's brain impedes the corresponding function of their consciousness). We're rather good at creating medication that affects consciousness in specific desirable ways. We can reliably induce physical brain states that lack consciousness, and most people have experienced such states (e.g. dreamless sleep or anaesthesia). The end of a consciousness (as far as we can observe) seems to correspond to the ceasing of brain activity. We have no reliable indication of the existence of consciousness that isn't tied to a brain and to brain activity. There aren't many things as complex for which we have this much evidence of reduction (although we can't strictly prove reduction or causation, but that applies to everything).

Some/most non-physicalists would say consciousness is non-physical. Dualists would say the physical and mental both exist as distinct fundamental categories, whereas idealists say only the mental exists. Some non-physicalists might just accept the existence of the other things listed instead.

Physicalists broadly reject the claims within points #3 and beyond, due to a lack of supporting evidence or justification.

Let's suppose sufficient justification is provided for any such claim (or if consciousness is somehow shown to exist distinct from the brain). Some physicalists might then classify such things as physical, especially if the justification provided is empirical evidence, i.e. what we can experience with our basic senses (which could be said to fit under point #1). Other physicalists might stop calling themselves physicalists and use some other label.

I don't want to know what things are physical, I want to know what the physical IS, fundamentally

We don't really know what the physical (or anything else) is, on a fundamental level. No-one knows what the fundamental nature of things are (even if some people act like they know). We can only observe the behaviour of things, through our consciousness (which we can't do much more than link, or not link, to things we observe). As an example, when we say things are made of atoms, what we're actually saying is that things behave as if they consist of specific parts, and those parts behave as modelled by atomic theory.

That's really the best we can do.

But of course our models might (or might not) suggest there are things beyond what we directly observe, and we might use epistemology to draw some conclusions beyond that.

Can we differentiate the physical from the non-physical?

As a physicalist, I only consider there to be 1 category of things and 1 place: the physical.

For proposed "non-physical" things, they either don't seem to exist, or they seem to be physical, i.e. they reduce to things that most agree are physical (consciousness seems to reduce to brain activity; people agree that the brain is physical, even if they don't agree that there is such a reduction).

Given what I said above, I don't see any coherent meaning of "non-physical", that would differentiate it from the physical in... any robust way, really, whether that's in terms of what things fundamentally are, or in terms of what we can observe.

Any way I can think of slicing things up ends up with lines that are little more than an arbitrary classification of things, with no robust criteria supporting that classification. Or it ends up with fuzzy lines that doesn't suggest a difference in the fundamental nature of thing, and it that classifies typically-considered-physical things as non-physical, or vice versa.

Most people would agree that invisible magnetic fields are physical, despite only being observable via it's affects on observable objects. But proposed gods are said to be non-physical, even when they supposedly affect observable objects (via miracles). Many say consciousness is non-physical, despite it being observable via brain scans and via its effects on someone's observable behaviour. This gets even more messy when we consider what our senses actually are (as per the scientific view) - you don't physically see an object, you see light bouncing off of it, and sight itself is light hitting a nerve in your eyes which sends a signal to your brain (or something). Direct sensory experience isn't technically actually a thing, at all.

Some might say that consciousness is a fundamentally different thing on account of us experiencing it, but:

• That seems more like a classification of subjective perception, rather than a different of fundamental type of thing. I don't see any reason to think it points to a fundamental type distinction.
• If you want to say that other people are conscious, you can only infer this via your senses. If we're following this line of reasoning, it would suggest other people's consciousness are a fundamentally different thing from yours, which rather undermines the idea that consciousness is a fundamentally different thing from the physical.
• "Fundamentally different"... from what, exactly? Your experience of all of your senses is part of your consciousness. It seems like drawing an arbitrary line to say your physical senses of physical things can produce conscious experience, yet your physical brain cannot produce conscious experience.

Answer 5

The problem is that there are several working definitions of what physics is.

Like "Physics", the word, is apparently Greek for "Nature", so everything that is in the realm of nature is in the realm of physics. Now depending on how broad you see that this can include... well everything or it could be the part of everything that is perceptible to us and thus informing our understanding of nature.

The first would posit that the "physical reality" is THE reality. The latter would open the door for a distinction between the natural and the super-natural (above the natural) or rather the "outside-of-the"-natural.

Also while the latter would technically allow for the physical and non-physical to exist, this is not really a hard demarcation line either. The problem being that this is a moving definition. So the non-physical wouldn't actually be defined in and off itself (so being described as it's own entity or by it's own properties), but it would just be defined by means of what it isn't, that is it's not part of the physical. Though the definition of the physical itself wouldn't really be conclusive either, but just consist of "what we can understand and measure so far".

So as long as the quest of science keeps going, we push that boundary of the physical ever further into the realm of what is currently still non-physical. Which is an odd idea, as that means the non-physical isn't so much an anti-physical or un-physical thing, but rather something that we haven't yet decided upon what it actually is.

And obviously for pragmatic reasons physics also defines our state of the art understanding of nature. So what is currently being able to be described by us, is physics and what isn't ... isn't.

Which puts physicists (researchers of nature), or at least the theoretical physicists, into a very odd position, because they'd largely not be doing physics most of the time, but actually ponder about things that are very much NOT PHYSICS (yet). Experimental physicist could still get around with "we're just asking nature questions", but they also largely can't get around having anticipations of what will happen, when constructing their devices, experiments or analyze the results.

You could also go a step further and move from physicists in the sense that encompasses all of science, to the researchers trying to find the principles of nature, so the underlying structure, the laws of physics that govern all objects and so on.

Which is similar to the theoretical physicists a rather un-physical approach as it relies on meta-physical (beyond physics) assumptions that such principles do exist, that matter/energy, momentum are conserved, that forces work bidirectional and whatnot.

Like those things do exist in nature or at least we can perceive things that appear to behave as if those things do exist in nature, but are these concepts themselves "nature" or are they outside of nature?

Or just in general are "ideas", "concepts", "thoughts" etc. nature or are they outside of nature?

Like another very intuitive definition of physics would be via the measurable effect and/or the interaction with physical objects.

So for example the more naive assumption could be a primitive materialism, so the assumption that everything there is, is of a haptic objectivity. You literally grasp things by grasping them and feeling their texture, weight distribution and whatnot.

However already the acoustic sensation kinda provides problems for that idea as you can't really grasp sounds. Now you can still rectify that one by arguing that sounds is a moving pressure wave, so technically still a material object, though you'd need to move from "just the directly perceivable by primary sensors"-physical objects, to a definition of "interacts with a physical object analogously so that the effect can be experience by primary physical sensors".

So idk you could employ dust or vapor to make the pressure wave "visible" for example.

Also while a lot of things happen based on interaction between particles, there are also descriptions of physical forces via fields, so physics moves from just matter interacting with other matter to being a description of the interactions itself. And there's also quantum mechanics where even the idea of an "observable" becomes less straight forwards and where we observe events only by probability distributions.

So yeah "interacts with physical objects" might probably one of the more general definitions.

Which actually makes the non-physical somewhat literally inconceivable because even the realm of reason, ideas and thought to some extend interacts with physical objects in that thoughts cause events and events cause thoughts.

Though even what persistently is hailed as the non-physical world, might at one point be conquered by the physical.

So a true non-physical would NOT interact with the physical reality or at most would be influenced by the physical reality, but with no means to respond to that even with the most minute change in the physical, because some day that could be measurable.

However if that non-physical has no implication on us at all, so that it doesn't even make a tend in our imagination, then for all intents and purposes we literally can (as we already do) ignore it.

Answer 6

According to Merriam-Webster's dictionary the word physical means

1. of or relating to natural science
2. having material existence: perceptible especially through the senses and subject to the laws of nature
3. of or relating to the body

For the philosophical question what is the meaning of 'physical', only the second lemma seems directly relevant. The first one would need to be based on an explanation of the second (otherwise it could quickly lead to an impasse). In the second one, the core meaning is indicated by

perceptible through the senses

The phrases 'material existence', 'especially' and 'subject to the laws of nature' create extra problems that are better avoided (again the problem of arriving at an impasse because of introducing yet another obscure term 'material' or because of circularity in definitions).

If I had to give a definition of physical, I would then propose

An object (or event, process, etc.) P is a physical object (event, process) if and only if P is observable through the senses, either directly, by our common senses, or indirectly, by using instruments (like a micrscope, telescope, etc) that facilitate direct sense observations.

The question then becomes: What does it mean to be observable through the senses? (Many -- I'm tempted to say: most -- philosophical problems are generated by the ambiguities of modalities expressed by words like "can", "could" and the adjectival ending "-able".)

Observable, I think, first of all means, what has in fact already been observed and reported as such, and has been accepted as such, by ourselves or by others. Having been accepted as observed by the senses (or by instruments that we understand sufficiently well to know them as extensions of our senses), those objects or events are also, in fact, accepted as what is (or was) real.

But observable as modal term seems to go beyond what is being or has been observed by the senses. Can we characterize this without making particular, contentious metaphysical or epistemological assumptions? I believe we cannot avoid all "future-looking" statements when further defining this, but those future-looking statements may hopefully be non-contentious. We can also not avoid referring to a group of observers.

Observable first of all means observable to an observer (or group of observers). If so, then for me (or for a group that I'm part of) the question "Can I observe object P?" will be answered as "Yes" if I actually have observed P or am observing P right now, but also as "Yes" if I accept that after taking certain simple actions, actions that I've often performed in the past, I will observe P. (The simple actions are actions like: "Look up to the sun, using these glasses." or "Look through this microscope at this droplet of pond water.") There is an implicit assumption of "normalcy" here, an assumption of having come to (being able to come to) intersubjective agreement in a group of normal observers making observations given a particular context. (A small child or a raving lunatic is not a completely reliable observer. Also, we may know that sometimes we ourselves are subject to sensorial mistakes, illusions, magical tricks, mirages, and false memories. A shocking example of this is that if we very carefully pay attention to something, this itself may corrupt our observations. You can verify for yourself by simply watching the following video while attempting to obey the instructions as well as possible Selective Attention Test.)

If the meaning of observable by the senses requires a reference to what normal people consider to be normal, it seems we have a problem. I believe we do indeed have a problem here, a problem that may even be insoluble if seen as a general, theoretical problem. I certainly know of no solution and would argue that in its full generality there can be no solution. -- For the sake of argument, let's assume there is no solution, that in a way, we're reached bedrock. The problem is How to determine whether a group of people is normal. For the sake of argument we accept that this is (or ultimately is) a metaphysical problem for which no general solution can be given. In other words, no decision procedure, no criteria can ever be found that always, unambiguously, justifiably give us a way to do this.

Even so -- both in daily life and in science, we do decide on this. We do judge, for instance, without first consulting psychologists or philosophers, that a toddler is not a completely "normal" observer -- for one thing, because being able to report on one's observations, not being so easily tricked, are also part of being "normal". If a toddler tells us they saw a ghost or saw Santa, we take it seriously as an expression of fear or awe, but not as sensorial observation. (I would however take seriously that they might have seen something.) (Some people on this forum may believe in ghosts, and may even believe they saw a ghost. If so, I regret to tell you that I cannot see you as a reliable, normal observer; I kindly refer you to Spinoza's amusing letter to Hugo Boxel about this subject.)

This implies, I think, that we can short-circuit the general metaphysical problem, by admitting

• (1) it's always possible that some observation was in error (or that we are not completely normal); but arguing also that
• (2) this does not imply that in fact any arbitrary given observation is in error (or that any recognition or denial of our or someone else's normalcy is completely unfounded).

In other words, it's generally reasonable to keep an open mind, and admit general fallibility, but we require that any challenge (or new concept) needs to be given in concrete terms, in terms of what is currently, actually accepted (what has been accepted and reported as observed). A completely general, abstract scepsis is irrelevant for empirical science and for daily life. (My opinion here is partially inspired by Wittgenstein's criticism both of abstract scepticism and of the abstract presumed refutation of scepticism in On Certainty.)

The most down-to-earth way to alleviate the constant, justified worry about making errors (or about not being "normal"), is

1. to insist on a controlled (or very well-known) environment in which observations are made;
2. to insist on repeatability of the observations, given that environment, and
3. to insist on intersubjective agreement, i.e. repeatability and acceptance by different observers.

This doesn't just apply to empirical sciences, but to also to everyday observations.

If all this characterizes "physical", then what would be the meaning of "non-physical"? In short: everything that is not observable by the senses.

Can we observe mental processes through our senses? Obviously not? Currently, it seems obvious, that we can not observe all aspects of any creature's mental processes. But we can imagine that it won't be that long anymore before we can create a brainscope that is able to visualize for instance what someone is seeing (or dreaming). Crude versions of this have already been made. Assuming that something like a brainscope can be made, even if it always would have some limitations, wouldn't this imply that mental processes, ultimately, are also just physical processes?

What about the soul, the supernatural, angels, demons, a God? What about miracles? The First Vatian Council puts an anathema on anyone who 'says that all miracles are impossible ... or that miracles can never be known with certainty', so we better tread carefully. But it seems clear that if someone admits than an object or event can in principle never be observed through the senses (or any instruments that extend the senses), then, surely, they agree that that is not a physical object. Scientists (or physicalists) don't have to deny that those kind of objects exist, they can simply remain silent about them. As to miracles and reported observations of miracles -- from a dispassionate point of view it seems strange that an institution devoted to the Truth, should find it necessary to curse and anathematisize those who have doubts about them. But if a miracle simply refers to a reported event for which we currently have no explanation, then it's easy to admit that there are many miracles. Scientific inquiry is never completely done -- and definitely is not done at this very moment. But this does not imply, of course, that any specific reported miracle actually happened.

Answer 7

In answering your headline question

What is the definition of physical?

I propose the following working definition: A phenomenon is physical if it can be investigated by the physical method. The latter is a synthesis of

• Observation
• Theory
• Checking theory against observation.

I try to satisfy your requirements 2) - 4). But I do not understand your point 1). What is the requirement here?

Added: This working definition classifies the following issues named in the OP's comment:

• What makes up the mind: Neuroscicence investigates the neuronal substrate of mental processes. Hence these investigations deal with physical phenomena.
• Qualia. I take qualia = felt qualities. I consider these subjective phenomena on the border between physical and non-physical phenomena. E.g., the experience of colours is a physical phenomena, starting with investigating the principles underlying the cones in the retina.
• A type of basically unobservable substance suggested in different interpretations of panpsychism: Apparently non-physical.

Answer 8

What is the definition of physical?

Aristotle took the natural world to be what constitutes physical phenomena. To that we can also throw in the ontologies described by theories that take that as their departing point: for example - energy, temperature, momentum etc.

He also included the first mover as a metaphysical part of nature. This was identified by early theologians with God. Note here that metaphysical does not mean of a religious nature but really the theoretical fundamentals of a theory of the natural world.

Is that definition clear enough to make a distinction between physical and non-physical?

But also minds are natural, being naturally embodied in human beings, but from the dualist perspective they are non-physical. However, Aristotle does not include mind in his categories of physical phenomena. On the very first page of his Physics he writes:

In any subject which has principles, causes and elements, scientific knowledge and understanding stems from a grasp of these, for we think we know a thing only when we have grasped its first causes amd principles and traced it back to its elements. It obviously follows that if we are to gain scientific knowledge of nature as well, we should begin by trying to decide its principles.

Here elements are roughly what you mean by physical phenomena. But he says first elements and so this means the fundamental kinds of physical phenomena. The term 'scientific' is obviously would not have been in Aristotles vocabulary but it is a gloss added by the translator as this is obviously what he intended by the original term. He then goes on to add:

The natural way to go about this is to start with what is more intelligible ...

That is we start with the natural world around us as that is most immediately "intelligible" to us

... And move from there to what is more clearer and intelligible itself ...

In other words, the theories we build to explain the natural world around us. And then:

... The things that are most immediately obvious and clear to us are usually mixed together ...

Like mind and matter as embodied in a human being

... their elements and principles only become intelligible later, when we separate them.

Like we separate mind and matter to understand each better. Aristotle obviously doesn't think mental phenomena counts as physical phenomena despite it being a thing of nature. But his decision has been ratified by later generations of natural philosophers who kept mind and matter apart. But why he did so isn't clear from this text.

Answer 9

I think this is more cultural (political) than objectively scientific. This narrow-mindedness, disinterest in big literature, big poetry, big music, big questions - is a hallmark of our time. 99% of big names in traditional philosophy would consider pure physicalism as dumb and uneducated. But nowadays, people are proud to declare themselves as consistent physicalists. This duping of the wider population requires simple ideas, simple questions and conclusive answers.

Answer 10

Since quantum mechanics is arguably the most successful theory in physics, I think it should be central in an attempt to define the physical.

The wavefunction of a particle represents all possible states it could be in.

If you have a system which only consists of one particle, then the wavefunction of this particle describes the entire system.

If you have a system of two particles, both having a wave function, you can describe the system with a multiplication of the two wave functions which will again be a single wave function that describes the whole system.

Now you could extrapolate this idea to the entire universe and imagine a single wave function that contains all info and interaction of every particle there is in the universe. That's the universal wave function

If we assume that gravity is quantised, and that the laws of quantum theory apply to all interactions, then the whole universe must also be considered a quantum system. As such, it must be described by a wave function. This wave function is then not only a function of the matter fields, but also of spacetime itself.

This would contain everything physical in the universe. So my definition of physical is anything that contributes to the Universal Wave Function.