Long ago I explained why All Godless Universes Are Mathematical and how physicalism already explains The Ontology of Logic and how numbers are not the same thing as quantities but only codewords that refer to them in How Can Morals Be Both Invented and True? And I’ve already explained why physicalism really means that every possible thing is reducible to some physical (as in, nonmental) thing in Defining Naturalism: The Definitive Account. But in my recent discussion of all this with Alex Malpass one thing we skipped for time was how actual infinities already exist everywhere. Which also raises other questions about how I think other weird mathematical objects exist. So I’m filling that in here.

The Background

I ascribe to a version of nominalism, the idea that all mathematical words and symbols and equations are just names for patterns of things out in the real world. And I am more particularly a modal structuralist, whereby all mathematical language describes actual or possible structures, whether causal structures or geometric structures or objects with structure and so on. All math is therefore just hypothetical discourse about possible realities. I’m oversimplifying, but getting more nuanced isn’t necessary here, unless you want to ask in comments. Generally, though, anytime you want to know my position on common philosophical disputes you can see How I’d Answer the PhilPapers Survey (and, for the questions they later added, How I’d Answer the 2020 PhilPapers Survey; though the first has my take on nominalism), and you can ask questions in comments there for anything not covered, or for clarification on anything that is covered. But here, let’s talk some Ontology of Mathematics (a subset of the Ontology of Logic).

Malpass took the position that nominalists require the physical (actual) world to have infinite resources. In particular he gave the example that unless the universe is infinitely large, we cannot explain actual infinities or how they work or how we can study them, because there can never be an actual infinity of anything. But he overlooked two things.

  • First: as I’ve often explained, not everything has to actually exist in nominalism. It only has to potentially exist. Thus we can make sense of an infinitely large world and derive conclusions about it because we can easily describe how it is a potential outcome of any actual world. Any space can be expanded endlessly, producing an endless amount of it. If you have one of a thing, you can have two. And if you can have two, three. And so on to every infinity. This is basically literally what Cantor did when he worked out how different cardinalities of infinity exist—because they “exist” as the potential rearrangements of any actual things. They did not have to exist as actual things for him to work that out, or to work out what the differences across infinities thus were. Hence we don’t have to actually make a thing to work out what would be the case if we did make it, just as we don’t have to actually grow a unicorn in a lab to know it will have trouble getting through our kitchen’s doggy door. Nominalism is always about hypothetical structures—and thus what those would look like, what properties they have, if realized. We don’t have to realize them to do that. But they do have to be realizable (in principle—as in, we can describe the resulting realization). Aristotle already made this point thousands of years ago when he invented nominalism. So we mustn’t overlook it as a fundamental feature of any effective nominalism.
  • Second: we don’t need a universe to be infinitely large for it to contain actual infinities anyway.

The latter point should not distract you form the former point, however.

Just because it happens to be the case that any finite collection already has infinite resources doesn’t mean we need that to be the case. Even were it not the case, potential infinities still exist in every required sense, and so all transfinite mathematics requires no further resource than hypothesized infinities made out of finite resources. Like infinitely duplicating a rock in your back yard. We don’t have to actually do that to work out what would be the case if we did: because that is inherent in the hypothesized state, and thus requires nothing more than that. This differs from straight fictionalism (another kind of nominalism) in that fictionalism imagines this has nothing to do with reality at all, whereas modal structuralism imagines that it is always anchored to physical reality in a crucial sense: every mathematical structure has to be physically realizable, using things that already do or could exist. It is thus an exploration of possible realities and constrained thereby. Thus every mathematical statement must be realizable in some physical structure to “mean” anything and thus be analyzable into anything we would call “knowledge.” Math is just hypothetical physics. So any world with any physics already contains all mathematical truths. Even a state of nothing does.

And that’s why physicalism fully explains why maths and logics can be developed that work and thus happen to so successfully and reliably describe reality. Any real thing is reducible to a mathematical description of its structure. Therefore every real thing in every possible universe is inalienably mathematical. There is no other way to have it exist and not be mathematically describable down to every necessary detail. Which means the converse is then true: any fictional mathematical structure will correspond to a world that can potentially be realized in the same way. There will be, in other words, some corresponding physical structure to every mathematical statement, such that that statement can never be realized without realizing the physical structure it describes. But we can still think about it even when never realized. Because that’s what we built math for. And the equivalency here is total, meaning, that is the sum total of the meaning of all mathematical statements. And therefore that is all math is doing: walking through all the potential rearrangements of the furniture of the world. Which requires nothing but any real world and the potential for it being physically rearranged. Platonism has nothing to do here. It becomes just angels pushing the planets—after Newton already proved the existence of gravity. We simply will never have any need of that hypothesis. While physicalism does all the work required.

Hence the title of this article is how all math is real—as in, in what way it is real. It is not that all possible mathematical objects actually exist somewhere; but rather that all possible mathematical objects potentially exist anywhere. Every mathematical object describes some physical object—and only carries any meaning at all insofar as it does. This does not require that the physical object described actually exist (yet or ever). It only requires that it could, as in, the object described, if fully realized, would be solely and entirely physical (geometry, quantities, ratios, causalities and all). This does suggest God may be logically impossible. But we have not yet proved physicalism true to a logical certainty, so we cannot be certain God is logically impossible. But we do have grounds to suspect it (see The Argument from Specified Complexity against Supernaturalism and The God Impossible).

Some Examples

You might say, well, “I have never encountered 326,519,438.004 objects” (although you just did: in your entirely physical, reducibly nonmental brain) “nor is there anything physical about it” (yet there is: the word refers to a potential physical fact: a specific ratio of things, which can be realized by simply adding more things to any smaller set of things, including some fractions of a thing as well, until you have the quantity that that number both refers to and can operationally substitute for in any language). Being a fraction (with that “.004” tacked on), it refers to fractional quantities, and fractional quantity is a fundamentally physical property: space, time, matter and energy all by definition possess it. It can refer to some actual fact (odds are, such a ratio exists between the respective lengths of some two objects in this universe—because there are so many objects in this universe to stand in ratio to each other) or a hypothetical fact (such a ratio can exist between two reducibly nonmental objects, without requiring anything irreducibly mental, e.g. I could cut two wires right now that have that ratio between them, just as I could collect 326,519,438 meter sticks and then add one 250th of a meter stick). And that is simply what that number means: the actual or potential physical ratio or fractional count of quantities it signifies (and thus can stand in for).

By analogy, there are no unicorns, either, yet the word “unicorn” still refers to a hypothetical physical fact: unicorns really exist whenever the physical (and thus reducibly nonmental) entity described by that word exists. And if they don’t “really” exist, they exist “fictionally,” since the thought of them exists in the brain that thinks it. And as long as the brain doing that is reducibly nonmental, then by the law of commutation, so is “the idea” of a unicorn. And even when no one has ever thought of it yet, the possibility of a physical arrangement of a physical brain corresponding to thinking it physically exists everywhere there is. And nothing more is needed than that. Just places to be, and what you could build there (see The Argument from Non-Locality). It’s not like if we flipped a switch on a strange machine that “turned off” or “erased” the Platonic realm we suddenly couldn’t think of a triangle or make one, as if some invisible force stopped us. No. Just having a brain that can compute things and a space to build them in is enough to have everything there is about triangles be true. Flipping that switch would do nothing. And so no Platonic realm is needed to explain anything at all about math or logic, or even ideas.

Thus if I have a gold ring, a gold cube potentially exists. Because I can mash it into a cube. But I require no supernatural power to do that. Nothing irreducibly mental need exist for a ring of gold to be potentially a cube of gold. Thus, potentially existing things are not irreducibly mental and thus not supernatural. Hence the same follows for a ratio like 326,519,438.004: if not referring to an actual ratio (like two actual wires, or the ratio of distances between two stars as measured from a third), then it refers to a potential ratio. And potential things neither are irreducibly mental nor require the irreducibly mental (as I discussed decades ago in Sense and Goodness without God, pp. 125-26).

This is why anyone who says anything like, “There is nothing physical about the fact that 4 is more than 3” is simply wrong. The fact that four sticks physically includes three sticks and another stick is a physical fact. And likewise any other instantiation of this pattern of physical quantity (of “four” things). The fact that this property can be shared by many things (indeed infinitely many: counts of apples, diameters of stars, areas of deep space, weights of rocks, eyes on a planet-eating space amoeba, etc.) does not require anything to exist but physical objects. In other words, there is no immaterial “thing” that you can “remove” from the universe and somehow make it impossible for “four sticks physically includes three sticks and another stick” and “four stars physically includes three stars and another star” from both still being true (and so on for every other possible expression of the same or any quantity). 

Hence we invented words to refer to that repeating pattern (which is a physical pattern, always and ever, even when only about hypothesized physical systems and not yet actual ones), which is, in this case, simply that “4 is more than 3.” That sentence did not exist a million years ago (humans invented it). Only what it refers to existed a million years ago. And what it refers to is the physical fact that any four things includes three things and another thing. Hence it refers to nothing else but the repeating physical pattern of “four physical things includes three physical things and another physical thing” (regardless of what kind of “thing” these are).

And Now for Infinities

One of the odd things we’ve discovered is that there are infinitely many infinities, each larger than the next. These are called “cardinalities” of infinity and are named aleph with a number, starting with zero (or “null”) for the smallest possible infinity, which is just the set of all countable quantities. But how can even that infinity “actually exist” (not even just potentially, but actually) in a finite world? Much less even bigger infinities? Well, prepare your mind.

  • Aleph-null = There are, right now, infinitely many geometric points on your fingernail. You don’t have to count them for there to already physically be an infinite number of them, right now. There are therefore infinite physical resources on your fingernail. Quantum mechanics makes no difference to this fact, as the functional fuzziness of spacetime at quantum scales makes no difference to the geometric properties of even those fuzzy spaces. There remains an actual infinity of geometric points on your fingernail. Physically. Actually. Right now.
  • Aleph-one = Then we add irrational (uncountable) quantities. Are there infinitely many irrational quantities on your fingernail right now? Yes. We could carve up your fingernail into all possible triangular areas, which thus include triangles with a ratio of sides equal to pi and every other irrational division of space. We do not have to actually draw these triangles: they are already there. Just as we do not have to count the geometric points to know there are, right now, infinitely many of them, so we do not have to draw every possible triangle on your thumb to know there are uncountably infinitely many of them. Those spaces exist. And there are uncountably infinitely many of them. Physically. Actually. Right now.
  • Aleph-two = Then we add permutations. So, if we draw an arrow from each of those triangles to some other triangle, there are infinitely many ways to do that—i.e. there are infinitely many ways to arrange all possible triangles on your fingernail into a sequence. Again, we do not have to draw these arrows, just as we don’t have to draw the triangles or count the geometric points. The sequences are already there, right now, on your fingernail.
  • Aleph-three etc. = Then it’s permutations all the way down. Just rinse and repeat: draw arrows from one complete sequence to another complete sequence and there are infinitely many ways to arrange those sequences. The quantity of all possible sequences of all those possible sequences of triangles on your fingernail is aleph-three. All possible sequences of all possible sequences of all possible sequences of those triangles is aleph-four; and so on.
  • Aleph-aleph = There are infinitely many permutations of permutations of sequences of triangles on your fingernail. Therefore your fingernail right now physically contains all possible infinities. And that’s even before we include potential structures. Even the finite space of your mere fingernail contains infinite things, and thus infinite physical resources to ground all mathematical truths about infinities. Physically. Actually. Right now.

You can say all the “actual” arranging here—the actual counting, the actual drawing of arrows—only potentially exists (no one has, or right now even could, do those things). But the counting and drawing is incidental. If there are three melons, there are three melons, regardless of whether you have counted them or drawn every possible sequence of arrows between them. So the count of melons really exists, and physically exists, as a quantity, even if no one ever counts them. Likewise their surface areas, and thus the sum of their surface areas, exists whether anyone measures them—–fully, physically, actually exists. Likewise there are six different ways to arrange those three melons (six different ways to draw arrows between them), even when no one has yet drawn any of those arrows. Every sequence of those melons exists, right there, all the time. Their very existence comes with the existence of every possible sequence of them. The actual arrows might not be drawn in the sand beside them, but you can stare at those spaces and realize every possible arrow is there. Drawing the arrows does not make something physically exist that wasn’t already there. You are not a sorcerer. When you draw different sets of arrows between those three melons you are simply acknowledging the physical, actual existence of those directions already being there, and you are just labeling them. So those six arrangements of those three melons actually exists—and could not but exist, the moment the melons do. And we need nothing more than the physical existence of hose melons for all that to be true.

Therefore, as for any three melons, so for infinitely many triangles on your fingernail: all actual infinities exist there, all the time—and could not not exist there. There is no way to have a fingernail and not have infinitely many spaces on it or there not be infinitely many ways to connect them. So all actual infinities—the whole infinite quantity if infinities—exist on your fingernail, indeed exist in every finite space. All worlds therefore have infinite resources to realize all possible mathematical structures. So this is not a problem for nominalism—at all, least of all modal structuralism. And likewise every other mathematical object. Some perhaps are not physically realized (like, say, an actual hundred-dimensional sphere) but all such are potentially realizable. All a hundred dimensional sphere is is a sphere with more dimensions added to the ones we already have, which we can physically describe, and which is potentially physically realizable—indeed, all “a hundred dimensional sphere” means is a physical object, a sphere, spanning a hundred dimensions of space, whether that space or that sphere ever actually exists or not.

And likewise every mathematical object.

  • Imaginary Numbers are physical descriptions of rotation in space (which we observe actually physically realized in electromagnetic systems).
  • Fermat’s Last Theorem describes any physical objects with sides in the ratio of an + bn = cn and the reason no such object with any whole measure above 1 can ever exist with n > 2 is that physical geometry makes all other such shapes impossible. For example, 23 + 43 = 8 + 64 = 72, and the cube root of 72 is ~ 4.16, which is not a whole quantity but fractional. And there just is no way to twist up the geometry of any physical space to get it to be a whole quantity—you always need “an impossible kind of elliptic curve,” a physical shape that can never be realized.
  • The Monster Group is just a description of a particular physical arrangement of a certain kind of system.
  • Number Theory is just a study of the possible physical arrangements and relationships of discrete quantities.
  • And so on. Markov Chains. Probability Theory. Octonions. Hilbert Space. Prime Spirals. Euler’s Formula and Identity. Always some physical system is being described (whether causal or geometric or anything else)—often one by now actually realized, but when not, always potential outcomes of physical procedures that no person or system has yet undertaken or undergone.

And likewise for every fiction, every possible world—like a world populated by unicorns, or solely filled with shrimp, or containing only one shoe, or where sentient jellyfish invaded Earth and gave us all universal income and healthcare. If physicalism is true, then the limits of what can be are indeed the limits of what is coherently physically realizable (on which see, again, The God Impossible and The Argument from Specified Complexity against Supernaturalism, as well as Defining Naturalism: The Definitive Account). And all it means to say that something is a possible world is that there is a potential physical way to arrange it to exist, that it could be realized, unlike Fermat polygons or square circles or solvable halting problems. Or purely fictional unicorns who can actually stab you. Or presently epistemically probable gods that aren’t evil.

So all actual mathematical and logical truths are accounted for by physicalism.

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