# Physical reality of physics properties

Is there an argument for the physical reality of physics properties? What I mean is the following: take force as an example. Suppose a tractor is pulling a plow with the same force as a nearby shed is pushing down on the earth. Physically, the two systems seem to have nothing in common. The only similarity involves human measurements and human speculations about what would happen if the situation were changed in certain ways (say by setting up a pully arrangement so the tractor is lifting the shed).

It strikes me as extremely implausible that such a thing as force is physically real in the same sense that the tractor and the shed are physically real. It's not even physically real in the same sense as the red color of the tractor and the musty smell of the shed. How can something be physically real if it only exists in the minds of certain humans?

A lot of the answers seem to think that I'm taking some sort of empiricist position: the force doesn't exist because I can't see or touch it. But my actual problem is not that I can't see the force; it's that force is characterized in terms of things that don't exist. Force is defined by it's potential to accelerate a mass, not by a mass that it is actually accelerating. Two forces are equal if they would accelerate the same mass by the same amount, but that's just a hypothetical mass and a hypothetical acceleration. Few forces we experience actually accelerate a mass by the amount the definition says it would. These hypothetical masses and hypothetical accelerations exist only the mind, so how can the force characterized by these possibilities exist in the world?

• There is a big distance from human measurements to "exists in the minds of certain humans". Measurements are physical interactions that produce identical damage from pulling by a tractor and pushing by gravity when the forces are measured equal. Getting hit by a brick is also a human measurement, and there is not much daylight between feeling damage and observing red in different places. The question reduces to realism about universals, and the standard arguments for/against apply. But many physicalists, Quine included, do favor irrealism about properties and relations generally. Commented Jun 3 at 17:06
• @Conifold, it's not just measurement; it's also that the properties are defined by possible events rather than actual events. Commented Jun 3 at 17:20
• @DavidGudeman I think you may be asking "are the mathematical objects used in physics processes or are they properties" not "are (a particular kind of) properties real".
– g s
Commented Jun 4 at 2:27
• Yeah. It is very easy to believe that the earth goes round the sun. Yeah... My school teacher told me so. How many must be alive today who can work out the heliicentric model from first principles? (To start with Galileo made his own telescope!) I say all this to remind: Yeah the earth goes round the sun — because my school teacher taught me... that X taught her who was taught by Y... And in all this morass of ungrounded hearsay I forget that every morning and every evening I have direct inconttovertible sensory evidence that the sun goes round the earth😁. Mutatis Mutatis for forces. Commented Jun 4 at 4:40
• Einstein was quite unambiguous about being inspired by Ernst Mach. You need to look at him -- especially his statement 3. Commented Jun 4 at 7:20

The force of a tractor pulling a plow or a shed pushing down on the Earth isn't one physically real thing (strictly speaking). It's a simplified model of a collection of the interactions of many, many tiny atomic particles (that also applies to the tractor itself, and the plow, and the shed).

And those atomic particles are themselves also potentially just simplified models of the things that are actually physically real, and they have charge that causes them to repel one another (which ultimately combine into some macro force), and that may also just be a simplified model of physically real things.

Science is models all the way down.

Science is in the business of explaining how reality behaves, not what reality fundamentally is, because we can't really know the latter. Everything we know about reality is ultimately based on observation, which relates to behaviour, so we model behaviour.

* I'm using "behave" broadly. A computer might behave in some way when you turn it on, and you can say you see what it "is" when you open it up. But a computer consists of components, and each component behaves in certain ways, and may consist of other things that behave in certain ways.

In any case, we may colloquially consider a tractor and a plow and a shed to all be "physically real things", and we may consider them to impose "physically real" forces on one another, but the reality of the matter (and the nature of our understanding of reality) is a bit more complex than that.

Disclaimer: I'm not a physicist, and my understanding of the nature of physical forces is limited.

How can something be physically real if it only exists in the minds of certain humans?

Newsflash. All physical reality only exists in the minds of certain humans. You might protest this is not the case, but deviant and pathological cases can be located in respect to all the senses. Let's take the case of the red apple.

Anyone walking into a room will instantly see a red apple sitting on the table because it is "physically real". Except, colorblind people. And blind people. Do we now say, how can the red apple be real if it only exists in the minds of certain humans?

But you might protest, and say that force is different than color and shape. But then I'd say, why is the sense of sight privileged over touch? Use a rope and try to hold back the tractor. What is the outcome? What is felt? Use a rope and try to pick up the barn. What is the outcome? What is felt? And more importantly, will you find any person who can stop the tractor or lift the barn? It seems that any able-bodied person who attempts these tasks will have ceteris paribus the same experience.

Force is as accessible to the senses same as color or sound or taste. But you might protest, the force is gone when the objects stop. That's not like the redness of an apple, to which I say, what happens to redness when the lights go out? All properties manifest themselves under certain conditions, and not others.

This recognition that external reality, happens to some extent in some minds in not others is a defining property of consciousness. It is the origins of phenomenology, and discussions about phenomena and noumena. It is the place where scientific realism begins to erode with the recognition that under a sophisticated model of thought, representations are not the things they represent. In this way, philosophical intentionality is understood as a construction. Indeed, it opens the door not only to constructivist epistemology like Brouwer offers but scientific constructivism.

The simplest answer to your question is as such. Physically real things are sensible and empirical and public to everyone. They persist whether or not we are aware of them. They are measurable. They can be described reliably. They are external to us and our consciousness. Forces may be invisible, but so is sound. They might not be objects in the sense of an apple, but they are readily detectable by spatiotemporal effects. This because processes (SEP) are just as real as objects, the prejudice of the Western canon cast aside. And both are part of the extension of the physical universe and participate as causes and effects.

• In terms of physical quantity definitions, I think we intuitively sense pressure (force distributed over an area) and torque (force acting at a distance from an axis of rotation) but perhaps we do not sense direct force. Turing a doorknob is a torque sensation. Sitting on a chair is a pressure sensation. Maybe the sense of muscle tension in certain muscles of the body translate directly to our concept of force acting like a vector along a line of action. The formula F = dp/dt applies for a body where external forces sum to zero then its acceleration is zero in an inertial reference frame. Commented Jun 4 at 16:56
• The biological model for our systems of perception and efforts to govern action in the sensory context (ego) is a central processing system with central pattern generators (CPGs), exteroception, interoception, and kinesthetic perception, with feedback control loops. The information arriving from the senses necessarily goes through some sort of cognitive filter that integrates the senses into our perceptions of the world. Physics models and perception models are concepts most likely generated by the CPG function of neurons. Reduction models don't get us to reality independent of our concepts. Commented Jun 4 at 17:08

What qualifies for you as "physically real"? We can see the tractor, and feel it, and use it to do work, but according to modern scientific theories, all of that is due to the operation of physical forces--the attraction, repulsion and interaction of subatomic particles. So in what sense, therefore, is the tractor more real than the physical forces that underlie it?

You could reject that, of course, for a more naive empirical realism that states "what's real is what you can directly sense," but that would force you to discard much of modern science. It's also vulnerable to the objection that the senses can be deceived, which underlies Descartes' Meditations, and which has been revived in the form of the "Simulation Hypothesis."

Even for the shed to have a musty smell, we are speaking of hypothetical measurements: if we walked into the shed and inhaled deeply, then we would experience a musty smell. The shed has the musty smell regardless of whether you do that, and the fact of it having the musty smell corresponds to what would happen if you did do it.

So to talk about anything physical, we must speak of measurements taken in hypothetical (also called subjunctive, or counterfactual) scenarios. In this, force is no different from a musty smell. The force exerted by the tractor is what we would measure, if we hooked up a force meter to the linkage.

Force is a physical quantity determined by men - not by nature. Force is defined

1. by fixing how to compare two forces,
2. by fixing the unit of force
3. and calibrating the value of force.

Classical example: One takes a a spring balance. 1) Force A is bigger than force B if force B extends the spring balance more than force A. 2) The force is 1 kp or 9.80665 Newton if it extends on earth the spring balance like the unit mass of 1 kg. 3) The force is 5 kp if the corresponding extension of the spring balance is 5 times the extension of the unit mass. This definition of force presupposes the definition of mass.

Defining a fundamental quantity like force does not give an answer to the question “What is force?” but to the question “How to measure force?”.

Newton's laws can be applied in one formula:

F = dp/dt

where net external force equals the change in momentum of a particle with respect to time. Force laws are recognized by the tendency to change momentum of the particle or system of particles.

The question is how did Galileo and Newton observe motion of bodies, invent quantities of measure, and in what way do we judge those models as attributes of reality rather than human measurement methods and math models?

In my case the human observer or conscious self was more or less a constant presence since early life long before I understood concepts of physics. I might have dropped objects from the highchair at the dinner table to watch them fall; and my mother probably did not approve of such behavior! So I learned some lessons about motion, free fall, external authority figures, and adverse moral judgments. Based on self contemplation in every context of social life I conclude that the self has attributes of the natural observer and the moral observer. To learn physics in the human context I also infer that Galileo and Newton had a human self similar to my human self.

If I hold that my models of nature or moral judgment are somehow real or objective then it is the assumption that in the absence of my self another human self would hold such knowledge; or in the absence of any human self those attributes would exist independent of my self knowledge. In my case since the self is always present in my conscious observations, and the self is never absent, I regard the attributes of reality which are independent of my self as The Great Mystery.

Even antirealism about force (a universal) doesn't get you,

"the two systems seem to have nothing in common. The only similarity involves human measurements and human speculations about what would happen if the situation were changed in certain ways..."

...as the nominalist position too declares there is immense similarity. It appeals to the physical similarities of the two systems to explain why force, whether real (universal) or irreal (nominal), is a useful description of the two systems. The human-dependence is very limited. It's the systems as they physically are, which lend themselves to "force" talk being useful.

So whether there really is a universal "force" which the two instantiate, or that the two, by their physical properties, lend themselves to similar descriptions, they really are similar.

So for your:

"It strikes me as extremely implausible that such a thing as force is physically real in the same sense that the tractor and the shed are physically real."

maybe you're just a nominalist about universals. But I see no way to escape the abject similarity of the two systems. That is, they share a tremendous amount of properties at issue, "force" is a useful and conventional term by the systems' natures, or they botch instantiate "force", etc.

To actually claim all similarities are in the eye of the observer is much too self-important and just inaccurate.

We don't know how things happen (we have theories about that); we only know that things happen.

We fill these gaps with forces/energy; and these forces/energy become physical attributes. For example color is the property (attribute) possessed by an object of producing different sensations on the eye as a result of the way it reflects or emits light (energy). Unfortunatelly, we account only for what can be measured.

It strikes me as extremely implausible that such a thing as force is physically real in the same sense that the tractor and the shed are physically real. It's not even physically real in the same sense as the red color of the tractor and the musty smell of the shed. How can something be physically real if it only exists in the minds of certain humans?

Force does not 'exist' only in the minds of certain humans. That's totally off base. You feel the force of gravity pulling you down to earth. If you are in a car that is accelerating, you feel a force. Thus your brain perceives/senses forces. So humans didn't invent forces, or force 'feelds'.

Only matter exists, so forces don't exist, but you ask are they real? I for one don't have a good definition of 'real' that can be of use in answering your question. But some forces are definitely perceivable, hence the invention of the word 'force' is justified.

The definition of force as F = ma, was a genuine leap in the understanding of how bodies of matter behave when they interact with other bodies. If you consider interactions as real, then since they aren't matter, you could consider the forces involved as real too.

Your objections arise from (at least) two sources:

1. Indirect measurement and experience: Most things we measure, e.g. velocity, momentum etc., can only be measured indirectly, but at least can be experienced directly. Some things, like force, can only be specified and experienced indirectly.
2. The modeling relation: Formal systems can accurately describe the behavior of natural systems, if the inferential entailment (of the formal system) is congruent with the causality of the natural system.

Some aspects of the real world can only be experienced indirectly: force being one of them. This doesn't make this aspect of the world less real than directly experienced things, like length or angle or velocity.

The entities of a formal system, like physics, have no physical reality; but such entities can make useful predictions about the natural world, because the formal system was designed to model the natural world. Only the properties of real-world things have physical reality: the entities of a formal system do not, since they just exist in our minds.

Indirect Measurement

But my actual problem is not that I can't see the force; it's that force is characterized in terms of things that don't exist.

Some things can be both measured and experienced directly, e.g. distance.

Others, like velocity and momentum, can be directly experienced/perceived, but are measured indirectly, e.g. velocity is distance per time, so you have to measure a length, then a time, then do a calculation.

Still other things are both indirect in how they are measured and in how they are experienced. "Force" is a concept that relates to the influence of the "environment" on a body, such that this influence changes the velocity (and so the momentum as well). Bodies certainly change their velocity in the real world, and so experience forces, where forces just mean "an influence that changes velocity".

But, you do not perceive a "force" directly, and this post goes into detail why this is so.

Feeling and pain in your body comes not from force acting on your body parts but from relative motion between body particles.

That's the key. It is never force, you feel. It is always only the effect of the force you can feel. It is the acceleration or relative velocity between body parts and particles you feel. Only. So the way we feel and experience forces is from how they cause changes in the otherwise equal motion of particles within us.

The Modeling Relation

How can something be physically real if it only exists in the minds of certain humans?

It can't.

But a concept (which has no physical reality) doesn't need to have physical existence, in order to make predictions about physical reality.

Robert Rosen described the "modeling relation", which depicts how formal systems (e.g. physics, mathematics) can model natural systems (tractors and sleds).

Essentially, when the causality in the natural system is matched by the rules of inference in the formal system, then:

1. you can encode information about the natural system into the formal system, and
2. decode predictions about the natural system, from the formal system

But my actual problem is not that I can't see the force; it's that force is characterized in terms of things that don't exist. Force is defined by it's potential to accelerate a mass, not by a mass that it is actually accelerating. Two forces are equal if they would accelerate the same mass by the same amount, but that's just a hypothetical mass and a hypothetical acceleration.

Your addendum changes the focus of the question to epistemic modality and the subjunctive mood in definientia. That's an intriguing question. But I think we can speculate using your example.

You say:

Force is defined by it's potential to accelerate a mass, not by a mass that it is actually accelerating.

First, we can observe something. While definitions are generally phrased in the present tense and completed aspect, there is no hard and fast rule that restricts the verb phrase constructed by the lexicographer. And a definition easily bleeds into an encyclopedic article. Both serve to function as real defintions.

Processes and objects are presumed to be persistent, and by their nature they hold certain properties to be invariant across time and space in the general case. However, there are conditions that affect properties. For instance, an apple would be red or green in the light, and not the dark. An apple would fall if released. When we start mincing words over possibility, then we are in the philosophical territory of occurrences and dispositions (SEP). Thus, the use of the subjunctive helps to remedy the gap between them.

Thus, the language you cite as part of the definiens of force is a way to specify the potential of an observable with the implicit caveat that there are conditions that falsify the proposition. For instance, the application of force doesn't always cause motion. Consider that an apple subject to the force of gravity doesn't move if it is in dynamic equilibrium with the normal force of the table it sits on. A definition that includes the subjunctive case to express epistemic modality, therefore alerts the reader to exceptions.

Thus, there is a metaphysical principle that seems to be at play. A more accurate definition includes conditions that impinge on being, particularly the nature of the properties that vary even though the object is held to be invariant. Furthermore, we can't conflate the properties of the occurrent hypothetical with the properties of the real object itself since objects are generally viewed as dispositions and not occurrences. Thus, a definition that includes hypothetical language to be applied to some particular circumstance doesn't impute the non-reality of the conditions to the object described by said hypothetical; such predicates define the hypothetical that features the definiendum, not the definiendum per se.

• The question is about the counterfactual of how the body changes its momentum under distinct conditions. Galileo originates the counterfactual reasoning. Based on his observations and quantity definitions he held that all bodies in vertical motion toward the surface of Earth would accelerate at the same rate in the absence of air resistance. And he held that all bodies projected at initial speed along a horizontal plane will move at constant speed forever in the absence of resistance. Newton refines Galileo's counterfactual to unite models for motion in the heavens with motion on Earth! Commented Jun 4 at 17:21
• @SystemTheory The question is about the definition of force. The OP wrote "force is characterized in terms of things that don't exist". Real definitions are an attempt to characterize real entities, and the OP is has noticed that counterfactuals are not real entities, but force is defined in their terms. The solution is that counterfactuals are not properties of real things they characterize, but of hypothetical contexts from which those things are to be understood. Your comment mostly appears to be an LLM-generated response IMNSHO.
– J D
Commented Jun 4 at 18:20
• @J D - I am a human being who did not consult an LLM or AI! But I doubt you would take my word for it based on your inherent bias! The counterfactual is the idea that a body of fixed mass remains in perpetual motion at constant speed unless acted upon by a net external force. When a body is at rest or remains in uniform motion then we must do a force law analysis, based on the surroundings, to determine whether there are no forces acting on the body or whether all the acting forces sum to zero. Then our knowledge of force laws is a mix of factual analysis also incorporating the counterfactual. Commented Jun 4 at 19:52
• ＠SystemTheory ：）I tease, of course. Sometimes I don't mind Poe's law well enough. Lo siento.
– J D
Commented Jun 4 at 21:16
• @J D - The definition of force involves the counterfactual where Newton had the good sense to extend Galileo's insight concerning the presence or absence of resistance. No doubt by expressing my own thoughts in my own words I am teaching LLM and AI how to imitate me which in some cases would be good for those who query AI and in other cases maybe not so good. Commented Jun 4 at 22:54

Tl;dr: Everything we take for real only has potential impacts.

You say that the tractor has physical reality but the force doesn't. The tractor is there, but the force is only a potential: It could accelerate a given mass by a certain amount, but that's purely hypothetical.

I challenge that distinction: The tractor is "potential" in the same sense the force is. If you don't interact with it, it could as well not be there.

You could run into it and hit your knee but you typically avoid that. You could put your foot under the tire and have it squished, but you typically avoid that, too. Oh wait, those were all forces. Ah well, never mind.

The argument you make is part of a much larger ongoing philosophical debate on whether the entities science talks about are "real." The camp of scientific realism argues that the entities science talks about, such as forces and gravity and energy, are real, with all the usual philosophical meaning behind the word. In their world, we are discovering truer and truer models of reality, which are eventually converging on the true reality. The scientific insturmentalism camp argues that science creates models of the real world, and those entities they talk about are not real. In their world, we are building models of reality, and it doesn't matter if "force" is real or not, as long as it models reality well.

So in the end, the question you ask is much larger - thousands and thousands of pages larger. And unanswered. There are philosophers in both camps.

Suppose a tractor is pulling a plow with the same force as a nearby shed is pushing down on the earth. Physically, the two systems seem to have nothing in common. The only similarity involves human measurements and human speculations about what would happen if the situation were changed in certain ways (say by setting up a pully arrangement so the tractor is lifting the shed).

The two forces you mention are different. They are not the same. They have a similarity, in that they both are described by a vector with the same magnitude, but they are not the same any more than an apple and an orange are the same because they are both fruits. The forces described are between different objects, and have different directions. The reaction force (the equal and opposite reaction) of the plow's force on the earth is a force upon the plow, while the reaction force of the shed's force on the earth is a force upon the shed.

It's not even physically real in the same sense as the red color of the tractor and the musty smell of the shed. How can something be physically real if it only exists in the minds of certain humans?

I find this argument somewhat ironic. Both the color "red" and the smell of "must" are concepts that only exist in the minds of humans. Both are very much perceptions. We could speak to the physics of light spectra or the chemical properties behind smell, but both "red" and "musty" are qualia, which only exist within a mind.

Force is defined by it's potential to accelerate a mass, not by a mass that it is actually accelerating.

Forces are not typically defined this way. True, the sum of the forces on an object does directly correlate with its acceleration. But forces have many other useful properties. For example a force over a distance does work, accelerating or not.

Few forces we experience actually accelerate a mass by the amount the definition says it would.

This is part of why we don't fully define a force in terms of potential to accelerate a mass. As an example, we can come up with situations where the forces are balanced, so there is no acceleration at all, but work is being done by the forces because there is a velocity in that direction. The work will be positive for one force and negative for the other force, but neither of those statements depends upon acceleration occurring.

• Force is not defined via work. Work is defined as force acting over a distance. If a body is rigid then it does not deform under applied forces. This is called the rigid body assumption. A coil spring is an example of a deformable body. The boundary work done to compress or elongate the spring is computed as the integral of the net applied force acting over a distance. If the position of the spring is not fixed in some way then a net force would accelerate its mass just like any other body. Work would equal the increase of kinetic energy plus any loss due to friction plus any boundary work. Commented Jun 5 at 15:19

### To start, physical objects don't exist...

A physical object is just a rational construct. Take a rainbow, for example. Physically:

• It exists only subjectively (you can't touch it, and it changes or vanishes depending on the position of the subject).
• You need to be of a certain size to perceive it.
• You can perceive it from a limited ground surface.
• It has no clear limits (meaning that everything in the sky is and is not part of it).
• A rainbow is made of water drops and light, which are also very difficult to define physically (light could be simpler to define than water drops).
• Physically, it changes constantly, however, you think the object you saw a second ago is the same you are seeing now.
• Its cohesion (existence and persistence as a unit) is an ideal: there is an amount of force (subjective) which will not destroy the object, and there is an amount of force that will destroy it. This means that if you blow with enough force, you can vaporize a rock.

You will say "a rainbow is a very special case". Ok. Let's try with an apple.

The only difference of an apple, a rock, a wave in the sea, a mountain or the wind, is the scale. All objects have the same physical properties, but different thresholds, depending on the subject (a small person would need to blow harder to vaporize a rock, for an atomic-size entity, apples can't exist, there would only exist atoms and galaxies made of atoms, which don't taste at all like an apple). Even a physical individual (a person) changes constantly, but you associate a single name with such phenomenon. When you speak with John, he is absolutely not the same than seconds ago. However, he is the same in YOUR mind for decades.

From a different perspective, we can say that all nature is in permanent change, there is nothing static (how can an object exist if it mutates constantly?). Statism is an effect of the mind, there is nothing static in the universe. In fact, that is probably the core function of the mind: to abstract reality.

So, all objects are essentially ideal constructs. Outside of your mind, everything is just a singular mess of phenomena, where parts cannot be physically discriminated (in order to calculate the precise weight of an apple, you would need to determine which atoms belong to the apple, and that is impossible, given that water molecules evaporate continuously... wait, is water part of the apple? if so, the streets are part of such apple, when it rains and it becomes wet...).

### ... physical objects don't exist, there are only rational objects...

So, where do apples exist? In our minds.

If I want to add 1+1, I need two PERFECTLY IDENTICAL objects, otherwise, in every sense, I will have something as 1+1.000000003982342... You might say, nonono, we should round that (true, you right!). But... to what significant figures? Oh, no! that is subjective! you will say 2, I will say 30000! But our minds, which have the innate capability of abstraction, there are just two apples, which are the same and that can be added.

Everything is just a tale in the mind, an unavoidable subjectivity, which nobody can escape.

### ... anyway, physical reality is just an agreement of subjectivities...

What we call objective is what is subjective but can be shared. For example, we can agree on a number of significant figures, certain conditions of temperature, humidity, air density, etc. and measure a table, and we can probably get the same value (such agreement is taken for granted, but it means not a physical reality). That is objectivity, something we can agree on within our subjective conditions of existence. But objectivity is really impossible, so it is far from being a physical reality.

So, forces, acceleration, etc., are just shared subjectivities, and can be considered objective, given that we can agree on sharing thresholds (a very fast entity would perhaps get a different measure of an object).

### TLDR:

What we consider physical reality is a shared set of subjective perceptions, conditioned by our "common"* biases. Now, is there something out there? What is really physical reality? We don't know. George Berkeley (empiricist) says only God is out there. Immanuel Kant says there is something that certainly affects our senses (called the noumenon), but it is impossible know it, we can only know what we perceive (the phenomenon). Science says yes (objects exist as such, that is called logical positivism or scientific realism), but only due to self-interest: allowing a minimal amount of subjectivity in scientific knowledge would make science destroy itself.

* Put within quotes since... what "common" would be, if there are no two equal objects in the universe?