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What is the relationship between the physical and our senses? If something is physical, must it necessarily be the case that we should be able to perceive it, at least in principle, directly with our bodily senses, or indirectly by expanding our innate senses with technology (e.g., the neutrino detector)? Can something be physical even if there is no known or conceivable way of perceiving it, directly or indirectly, with our senses?

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    Can you give an example of something that you consider non-physical?
    – JimmyJames
    Commented Mar 11 at 15:36
  • "no known or conceivable way" I feel like this would be a better question if this phrase was replaced with 'no possible way'. But I might be misinterpreting your question. Are you asking whether physicality depends on human perception? Kind of a 'if a tree falls in the woods...' sort of question?
    – JimmyJames
    Commented Mar 11 at 15:42
  • @JimmyJames The question was prompted by the comment section discussion under this answer. As for examples of non-physical things, that's a point of contention. This answer offers some examples, such as consciousness (purportedly).
    – Mark
    Commented Mar 11 at 16:00
  • Would the universe during the cosmic dark ages fit the definition of "no known or conceivable way of perceiving it, directly or indirectly"?
    – JimmyJames
    Commented Mar 11 at 16:21
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    "Physical" evolved by extension from "ponderable matter" of everyday objects, to micro-particles and then fields. Accessibility to senses (with repeatability and public agreement on the outcomes) was only one of the extension conditions, subject to consistent interaction laws was the other. As light cones and parallel worlds suggest, we are ready to give up the first. But ghosts, qualia and abstract objects (as interpreted by realists about them) suggest that we are not inclined to give up the second. Their behavior is "too far" from "ponderable matter" even if they are "sensible" somehow.
    – Conifold
    Commented Mar 12 at 0:09

7 Answers 7

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Can something be physical even if there is no known or conceivable way of perceiving it, directly or indirectly, with our senses?

There could be completely undetectable physical things - we just wouldn't know about them.

The closest thing to undetectable that we do know about is dark matter - it's only mostly undetectable, it has mass so it affects gravity.

There could be undetectable parts of physics that don't interact with the particles that make up our every day life and don't have mass, that would be maybe 100% undetectable. There might be analytical ways to infer their existence, but there's no guarantee.

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  • I would argue that "dark matter" is undetectable as it is only a theoretical possibility (the other possibility being some equation is wrong, or sth else being the case). So it is only theoretically conjectured to exist
    – Nikos M.
    Commented Mar 13 at 12:12
  • @NikosM. I suppose you could say that, but of course it's still worth pointing out that we think it exists because of things we have detected, which means that if it exists, it's detectable - but only just, and not very well.
    – TKoL
    Commented Mar 13 at 12:14
  • I would not count the theoretical reason we think sth like dark matter can exist as a detection of it..
    – Nikos M.
    Commented Mar 13 at 12:16
  • @NikosM. right, that's not exactly what I said though.
    – TKoL
    Commented Mar 13 at 13:03
  • @NikosM.: All proposed explanations of dark matter either interact with regular matter in some way, or stipulate that our understanding of physics is incorrect such that dark matter is not required to exist. There is no such thing as totally non-interacting dark matter - nobody has seriously proposed such a thing.
    – Kevin
    Commented Mar 13 at 16:57
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Referring to perception is an erroneous restriction. We can't perceive planets in far off galaxies, but that doesn't stop there being planets in far off galaxies or make them unphysical. Physical things interact with other physical things, but might do so very weakly or infrequently, so that any form of detection is a challenge.

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  • I agree, of course. But how would you define "physical" and delineate it from "unphysical"?
    – Jo Wehler
    Commented Mar 11 at 20:53
  • @JoWehler Hi Jo, I hadn't thought about it, but will ponder and get back to you if anything springs to mind. I wonder if it is possible that the only definition is that physical things are things we consider physical. In other words you would delineate physical by naming all the categories of things considered physical. Commented Mar 11 at 21:22
  • For example, we can mention matter, energy, fields, forces, spacetime etc and say they are physical, and everything else is unphysical. Might that work? Commented Mar 11 at 21:24
  • @MarcoOcram I would think the delineation is that unphysical things cannot interact with each other. Though you could argue that my unphysical thoughts about your face might interact with your unphysical thoughts about my friendliness via my very physical fist.
    – DKNguyen
    Commented Mar 11 at 22:52
  • @DKNguyen exactly. Commented Mar 12 at 6:32
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Where exactly do you want to go with that question?

I mean if you take our current level of understanding and regress to a time with insufficient technology it's quite easy to imagine for example that radiation is a physical effect, that is impossible for us to perceive directly and quite hard even to detect indirectly as the effect is often quite small and easily overlooked. So it would be something physical that we couldn't directly or indirectly perceive and you could imagine that there are other things like it, that we have yet to discover.

On the other hand if we can't see them, can't hear them can't perceive them in any way shape or form and don't see their effect by any of those senses, how should we know that they are there in the first place? Like it's not the "being physical" part that is the problem but the "BEING" part in the first place. How would we even know that we don't perceive them if we don't perceive them?

Now the "obvious" counter might be the existence of particles which were predicted in theory before they were discovered in nature, for example because a calculation showed a violation of conservation of momentum and/or energy or such fundamental relations that we believe to be fixed and which we therefore assume to must hold even if our observations don't readily confirm that. And where we thus assumed that something is there even if we didn't see it's effect (yet).

But there are two problem here a) the (yet) part so does it count as not being physical or are we just not aware that it is and b) wouldn't we have already seen the effect of that thing with our senses indirectly? Is contradicting our logic already perception enough or does it need to be a primary sense? Like after all we do consider measurements done with a computer to be primary enough so not sure it's valid to discard that.

So yeah in order to categorize something as physical we need to be aware of it's existence in the first place and for that we need to see it's effect on us and/or our environment (by our senses). So yeah that is kinda a requirement. On the other hand it's perfectly possible for things to exist without our knowledge and it's kinda arrogant to make our perception a requirement for their existence.

So idk consider a mysterious field, modifying a mysterious property that has an effect that we can't see but that our alien neighbors could see. If we don't see it, then we don't think about it, it takes up no space in our environment and we don't consider it physical because we don't consider it at all. Once we meet those aliens and they tell us about it, we become aware of it's existence and it suddenly becomes physical (or always was) due to the fact that we can know indirectly perceive it (by communicating with the alien via our senses).

Though what if the alien is lying? Are we then perceiving something despite the fact that it doesn't exist? Do the concept of "lies" and in general "impressions" become physical because they can be perceived? The thing is our senses do often confuse the cause and effect of a an action. Like a sound or picture isn't scary, it's just the effect that the object has upon us, though the cause of that effect might very well be physical and measurable.

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    "Once we meet those aliens and they tell us about it, we become aware of it's existence and it suddenly becomes physical" I would consider that a 'conceivable way of perceiving it'.
    – JimmyJames
    Commented Mar 11 at 15:39
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Two cents.

"Physical", in most cases, means "being a form of matter-energy, or a property of matter-energy (eg charge, or a conservation law)".

Matter-energy can be such that is detectable or sometimes undetectable (eg a form of "dark matter").

Even if undetectable though it can be inferred to exist due to other reasons (eg the discovery of a new planet due to anomalies in orbits of known planets and known laws of motion).

So the answer is no, something physical need not be directly or indirectly perceptible. It may not be perceived yet still be physical.

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  • Isn't inference a form of indirect perception, though? Using a device to detect something rather than direct sensation is itself a form of inference, after all; we infer the presence or properties of something rather than being personally aware of it because of the response of the device.
    – Idran
    Commented Mar 12 at 14:16
  • In what sense is the inference that sth like dark matter must exist (due to some theoretical reasons), a kind of perception of dark matter?
    – Nikos M.
    Commented Mar 12 at 14:22
  • I was responding to "even if undetectable though it can be inferred to exist...". I'd consider the sort of inference you describe a form of indirect detection/perception; that is, I wouldn't consider the word "undetectable" accurate in that case. We describe exoplanets whose presence we only know of via gravitational effects on its star as "detected", after all.
    – Idran
    Commented Mar 12 at 14:25
  • Ok what you say can be taken as an indirect form of perception, but I don't want to limit the inference only to these cases. Dark matter is inferred to exist purely on theoretical grounds.
    – Nikos M.
    Commented Mar 12 at 14:27
  • Isn't it also inferred to exist via the required gravitational mass of the galaxy vs. the total amount of perceived visible matter? Dark matter is literally just "the matter we can't see that contributes to the gravitational mass", after all, whatever form that takes.
    – Idran
    Commented Mar 12 at 14:29
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Ultimately this comes down to the question of what 'physical' means. This is a not a simple question. As mentioned in Section 4.2 of this article on Physicalism:

Along with the concepts of space, time, causality, value, meaning, truth and existence, the concept of the physical is one of the central concepts of human thought. So it should not be surprising that any attempt to come to grips with what a physical property is will be controversial.

The idea that physical means something that can sensed directly or indirectly is a fairly common interpretation but not the only one.

I would also make a distinction between something being 'physical' and being 'real'. A planet is an inherently a physical thing. What about a hypothetical planet? Is that physical? If it existed, I think we would all agree that it was physical. Another way to look at your question is whether being real is a necessary condition in order to be physical. But that leads to another question of what it means to be real.

I don't believe anyone from any side of these debates has conclusively defeated the other arguments, but I lean toward the idea that 'physical' means something of or relating to things that are detectable by our senses. By that I mean, for example, that the lightbulb is an inherently physical concept. It's defined by its physical properties and even the idea of a lightbulb is physical. In other words, you cannot separate the idea of a lightbulb from its physical definition.

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  • What of the thought bubble lightbulb, the new idea being lit up?
    – civitas
    Commented Mar 12 at 0:52
  • @civitas I would say that's a metaphor based on the concept of a literal lightbulb.
    – JimmyJames
    Commented Mar 12 at 15:01
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What is physical can mean different things when viewed from different perspectives. But I would try to give the highest level qualification.

Let's say we live in a computer simulator (the Matrix), a bunch of programmers and architects are actively maintaining this system.

Then, everything falling under the rules of this simulator would be - physical.

But most of the exceptions to the rules, a code that does not apply to the simulator's inhabitants, something reserved for God, like programmers entering the simulator - would be non-physical.

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  • The director then yells: "cut print it".
    – civitas
    Commented Mar 12 at 1:16
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No. Qualia (for instance) is directly observable but not physical.

In general, for something to be physical it should be observable with an approach that satisfies the criteria of scientific method. This (among other things) requires observability by different observers who can communicate their results to each other.

So, something is not physical if:

  • After observing it the observer inevitably dies before being able to communicate it (say,occurs beyond the event horizon of a black hole).
  • After observing it, the observers cannot communicate to each other (for instance, get separated into parallel universes)
  • After observing it the observer gets duplicated or split in such a way that each copy reports different observations
  • The observer has no suitable language to communicate the observation (for instance, new smell sensation)

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