Just for utility’s sake, I will organize some of my past comments here on identity theory. By that here I mean one’s ontological model of “identity” (not sociocultural identity): that which makes one thing different from another, or unique among all others. I have addressed this before (along with the related question of whether we can survive being teleported), but perhaps too briefly.

Sameness vs. Identicality

Identity is causal continuity across a numerical unity. If you have a numerically singular thing with a continuous causal history, it is by that fact a distinct thing (it is the same thing across time, essentially a four-dimensional tube). If that thing then spits into two numerically distinct things with their own separate causal histories, then they become distinct from each other, and are no longer “identical” but could possibly (and at least for a moment will) be “the same” in a different sense, which we can call “plenary identicality,” being the same as something else without being identical to it. In Aristotelian terms, being the same as something else is sharing a “formal” or “pattern” or “abstract” sameness, whereby a universal property will satisfy Aristotle’s first law of logic (A = A), like triangularity or being a book, while being identical to something else is that by which a particular thing will satisfy Aristotle’s first law of logic (A = A), like a particular triangle or book (on the physics of particulars and universals, see Why A Neo-Aristotelian Naturalism Is Probably True). So, per the Masked Man Fallacy, all masked men are the same in a plenary sense (they are all “masked men”); but only one actual man is the same as any particular masked man.

Which means for “personal” identity (being the same person), “biology is incidental (rather like what hardware I’m running my word processor on: it doesn’t matter to the content of the book I’m typing).” Personal identity is a mental pattern with a numerically unique causal history. Because The Mind Is a Process Not an Object. Thus, “if I woke up tomorrow converted into an electronic body and brain,” but still with all the requisite patterning the same (memories, skills, proclivities, etc.), “I’d still be me, however altered.” This is because we have one physically-demarcated entity here (my mind does not physically or experientially overlap anyone else’s—contrary to the expectations of Idealism but entirely expected on physicalism, as I discussed last month). And it has a singular causal history. Because the “electronic” me was caused to be arranged as it is by the “biological” me, by whatever process of collecting the data from the one and imprinting it on the other.

Which is distinct from accidental duplication. If a brobdingillion years from now a Boltzmann me forms by pure chance accident, it will not be me in this sense. We will not be identical because we do not share a causal history. Indeed, if this Boltzmann me formed today, we won’t even share numerical identity. There will be physically, distinguishably two of us—and our minds will not overlap (I will not know what he is thinking, or vice versa, and causes henceforth impacting me will not affect them, and vice versa). So if that happened, they’d obviously be a different person, just similar (even if precisely identical for a second of time, our causal histories will immediately begin to diverge). They wouldn’t even be a “copy” of me in common parlance, as a copy entails causal relationship (the copy is caused to be in its arrangement by some sort of causal interaction with the original); and there would be none here. So if they would be a different person now, so would they be if they formed a brobdingillion years from now. Hence particular (as opposed to plenary) identicality requires both numerical identity and a continuous causal history. (And if you ask “Why,” the answer is, “Because that’s how we use that word in ordinary English in this case.”)

The same would follow for another universe otherwise identical to this one. If both started independently of each other but evolved identically, that’s just Boltzmann universing, and thus we do not have numerical identity, but two separate things. Their causal histories may be identical in one sense (as in, sharing the same arrangement) but not in another (they are numerically different and causally unconnected with each other). Hence they will not be wholly the same—because that’s what “numerically different” and “causally disconnected” mean.

For example, every copy of my book Sense and Goodness without God is “the same,” because it is identical in that sense (plenary identicality), but it is not “wholly” the same (particular identicality), because each copy has a different physical location and causal history (burn one, and the others won’t miraculously catch fire; annotate one, and your notes don’t magically show up in any other copy—note that these kinds of things can happen to electronic copies precisely because those maintain a singular causal relationship). So even if two universes share a past causal history—for example, if both are offshoots of the same Big Bang, with exactly the same deterministic sequences and conditions, so that, though numerically distinct, they proceed identically, all the way to causing a “me” to exist at precisely the same corresponding time—those two versions of me will not share any causal history (that shared causal history long predates our existence in either universe; hence neither of us can access the other’s mind nor make decisions for them). So we will be different people, even if (like copies of my book) we are “the same person” in a plenary sense.

Sameness vs. Change

Of course we often talk about someone being the same or a different person in a different sense than that, to refer to how someone might have changed (or not). Likewise with objects. The Millennium Falcon is not the same ship she once was. A digitally streamed book can be edited (propagating instantly to all extant copies of it), and thus no longer be the same book it once was. Yet at the same time, these things are still the same in a broader sense. The Millennium Falcon, however changed, remains the same ship across time; as distinct from all the other YT-1300 Corellian light freighters, even those that just happen to also be named “Millennium Falcon.”

Likewise, an edited book is still in some sense also the same book. If I electronically propagate a revision of all its typos, my book Sense and Goodness without God will not become Lord of the Rings or a manual on nail care or even any of my other books. But at some point, enough edits, and my book will be a different book (which gets to the Sorites Paradox). But when that is will depend on what you care to mean by “different enough to not be the same anymore.” For example, we can observe extended cognition that even blurs the lines between where your body actually ends and the external world begins, whereby your tools, your library, and your country are all extended “parts” of your mind. But you have to choose to draw those lines. The distinction between your body and your tools, your library, and your country remains physically just as real. So it’s simply a choice by you of which boundaries to talk about, which can be constrained by what you need to know (e.g. “Am I in Chicago?” is not going to answered by defining “you” so as to include your library or your country).

Which brings us to Demarcation Theory and the Theseus Paradox, which I have already covered in How My Philosophy Would Solve the Unsolved Problems. As I summarized there:

We can demarcate individual objects any way we want—e.g. we can “choose” how to tell one mountain apart from another when they are interconnected, by simply choosing what we mean by “individual” mountain, since this is simply a question of why we need to tell, i.e. what use is it to know whether we are on Mount Pinos or San Emigdio Mountain? What difference does it make? And in particular, what difference does it make that we care about?

Most individuation starts there: choice. Which is always a function of need or purpose. But there are also scientifically factual differences to key on. Where we choose to demarcate individuals is arbitrary, or purely utilitarian; but that those demarcations exist is a physical scientific fact. And it is decided by location in space-time and divisibility in principle. No matter how we draw any boundaries between them, as long as we do draw any such boundaries, Mount Pinos is never in a physically identical location to San Emigdio; and even if they could be (if, like photons, they did not conform to the Pauli exclusion principle), they would still be conceptually separable (as photons passing through each other are), retaining their separable properties and histories—unless they really did merge completely into a single mountain, such that there was nothing left to demarcate them by. All of this follows from observing ordinary language in practical, real-world use. 

So, when does something “change so much” that it becomes a different thing? The answer is always pragmatically arbitrary, and follows from why we care—what is it that we want to know? There is certainly a categorical change from a cell to a fetus and a fetus to a baby and a baby to a child and a child to an adult. But we can even say that with the movement of a single atom, at every instant the cell exists it is a different cell, and every instant an adult exists they are a different person. And that is true. But it is not true in any sense we care about.

For example, when we ask whether Joe is the same person who killed Jane, we are talking about causal continuity and numerical identity, which entails Joe Now will have memories, for example, of having been Joe Then killing Jane (and even if not, there will still be an objective ontological fact of the matter that we could, in principle, trace in molecular or timespace “memory,” since that is what it means to Joe Now and Joe Then are the same person in that sense). “But he’s a completely different person now” can also be true, it just won’t be relevant to this point, because “different person” is then meant in a different sense.

That Joe is a different person from Jane’s killer (and thus some other culprit killed her) is causo-historically relevant in a way that “Joe has changed in his character” is not—though the latter might be relevant to his sentencing, it won’t be relevant to his conviction. So we can have different senses of “same” or “different” that we care about. But we do have to specify which we mean before we can answer the question of whether it’s “true” that someone or something is the same or different. For example, “Joe has changed in respect to his character” can be either true or false; and it will be true or false irrespective of whether “Joe is the same person who killed Jane” is true or false (as it, too can be true or false, just in a different sense, because it means a different thing—a point you might more readily get if you read My Monthly Recommendation: Ayer and Polanyi).

So the extent to which someone is a “different person” is a function of what you mean when you ask that question. The answer can also sometimes be blurry, when very it’s close to your chosen demarcating line. But usually it’s clear enough, such that one can be both the same person and a different person at the same time, on different senses of each. Robert Downey Jr. is the “same person” today as starred in films in the 90s in the strictly causal-historical sense; but he is also a “different person” today than then insofar as he was then a reckless drug addict and today is not. Even though there happens to be a biological platform those changes and similarities are being “run” on, the changes and similarities are only relevant in respect to their psychology (who Robert is “as a person,” the pattern that defines his mind). If, unknown to us, Robert was converted into a robot in 1998, none of those things change: he still is the same person who starred in those earlier movies and is still also a different person now in respect to his wisdom and character. But now he will also be a different person in an entirely new sense: Robot Robert is “different” from Biological Robert in respect to the physics of his body.

So we can choose what degree of “sameness” matters for any given query, as we please. We merely need to avoid equivocation fallacies when switching between different thresholds of “change.” Thus, we know that Robert Downey Jr. can survive a trip to Paris but not a trip through a meat grinder (since that destroys the pattern that distinguishes him as a person); yet at the same time police investigating his death will correctly affirm that that is Robert Downey Jr. on the other end of that meat grinder (albeit dead), because they are referring to a different sense of “the same,” one that is relevant to what they are doing. But we cannot then equivocate our way to saying that, because we can converse with Robert in Paris (because Robert here is “the same” as Robert there), we can therefore converse with Robert’s ground-up corpse (because dead Robert is “the same” as live Robert). These are different senses of “the same.”

The Teletransporter

With quantum mechanics we know the quarks composing the nuclei of the atoms of your brain are being swapped out with virtual quarks all the time, so that from one second to the next what theorists speculate in teletransporter problems has actually happened to you already—all the core matter of your body is “disassembled” and “reassembled” every split second (it just happens at a Planck-scale in time, so none of that happening is easily detectable to modern instruments). So. Do you consider yourself a “mere copy” of your one-second-ago self, or the same person? And why?

No one ever really stays exactly the same, as if frozen. So identity is never, in practice, about being frozen in a single pattern, unchanging and insensate. You gain memories, knowledge, skills, wisdom, traumas, disabilities, age—and change location, and between awake and asleep, and between joyous or bored. So in ordinary parlance, you remain the same person not by being “exactly the same pattern” from one time to the next, but by being in a causal chain of past states of that pattern, each causing the next. So as long as your past pattern-state caused your present pattern-state, those pattern-states, past and present, are the same “person,” even as they are also a different person in various other senses. In four-dimensional geometry, you’re a single identifiably distinct “person tube” spread across time, but that tube also changes over time. Yet remains a singular distinct tube.

In the transporter case, your past pattern organization causes the buffers to code a distinct output, which continues you on in a compact unconscious state (a “record” and then a “transmission”), which eventually causes the new assembly at destination. Thus, causal history is maintained. Just as with resurrection. Someone whose brain after dying is frozen in cryo and then resurrected nanorobotically centuries later remains the same person, they are numerically identical with a continuous causal history. Events change us while we are alive, every minute of every day, and thus so would death and resurrection, but whether they change us so much that we cease to be “the same person” will depend on what you choose to mean by “same person.” That is therefore a semantic, not an ontological question. If you are asking whether the matter or the pattern matters to particular identity, then only the pattern matters. Theseus’s ship remains Theseus’s ship even if every plank is replaced, so long as they were replaced in situ and thus the past ship caused the present ship to exist, and remains numerically identical to it. What the ship is made of is irrelevant, only its pattern and causal history matter to what most people care about most of the time.

Hence likewise what your brain is made of is irrelevant—as proved already by quantum mechanics: your brain is never made of the exact same stuff from one moment to the next; nothing about you is. Continuous quantum quark-swapping means the protons and neutrons comprising the matter of your brain and body is always different. We see a continuous neutron, a continuous proton, but really neither enjoys real continuity: like Theseus’s ship they are being swapped out with new neutrons and protons gradually over time. It’s just that the time scale is so small we can’t “see” this (and thus don’t really care about it), just as cinema films are really just frozen cells broken by black bars every 24th of a second—but that’s too fast for our eyes to see, so we “see” a continuous image moving on the screen. But it isn’t continuous. And nothing in it is moving. There are gaps as one cell is swapped out for another cell; it just happens too fast for us to see it happening. And the new cell differs from the last one in just such as way as looks like movement. And in one relevant sense, it is movement. Just of a different kind. A car in a cell of film is not numerically identical to the car in the next cell that is an inch now further right; but our brains are meant to understand the same car as moved. It’s a fiction. But one we accept and understand. The fiction represents a reality: of a numerically identical car moving down the road, with a singular continuous causal history. Film tells a story. It is not identical to it.

So imagine instead of a few Planck times separating the disassembly and reassembly of all your atoms, we built a machine that could “slow time” and thus “pause” any moment where your atoms have dissolved into a “quark sea” and “hold the pause” as it were for a much longer time (say, a day); and suppose for the purpose this machine can wait the few split seconds it takes for all of your atoms to undergo this sea change (since this means individual atoms changing out slowly bit by bit over time, not all at once, so it will have to hold each one as it dissolves, one after another, until all of them have done it). That machine would be doing what any imagined teletransporter does (disassembling you and transmitting the data and reassembling you). Just imagine we can then “move” that machine across the country in that day, and then we turn the machine off, and all your quark-sea’d atoms collapse back into new atoms and you appear—where you would have appeared naturally a split second later, only now it’s just been stretched to a day, and some distance has been traveled.

Thus you have teleportation by disassembly and reassembly by doing nothing whatever but pausing how long the gap is between the natural continuous disassembly and reassembly of your atoms going on all the time. Why would that be any different than it taking a split second and crossing a fraction of a meter of distance as you travel in a car, say? Why would the “amount of time” in between when your quarks go away and are replaced with entirely new quarks make any difference to the continuity of your consciousness? And that’s all this machine did: stretch the amount of time. The rest is just what naturally happens to you all the time, every second, of every day. You are not made of any of the same stuff by the end of any day than you started with. It has by then all been swapped out. You are Theseus’s Ship.

That’s not even theoretical. It’s a known fact of physics now. So it cannot be that “stuff” has anything to do with who you are and what maintains your continuity as a person. You are not an object. You are a process—a pattern, that is maintained by a process, and that experiences as a process. And a process can be sustained by any underlying stuff. The stuff is just the means of generating and maintaining the thing; it is not the thing itself. And a process can be paused (sleep, feints, comas) and continued, and remain the same process. Thus, consciousness (and personal identity) are irrespective of the underlying machinery recording and generating it. The machinery can be swapped out seamlessly, without you even knowing it (and in fact, per quantum mechanics, is swapped out seamlessly, without you even knowing it). The process remains the same. And therefore, so do you.

But what about Thomas Riker? At one point in Star Trek TNG Will Riker is accidentally replicated and the two Rikers develop as different people. So, are they the same person? In one sense, yes. They were numerically identical with the same causal history up to a certain time, and for that very reason remain very much alike even after they split. The teletransporter in this case accidentally built two Riker’s, one each in a different place. The two then did not share a brain and thus did not share a mind. They could not think or feel what the other does, nor control their counterpart’s body. They are different people in the strict sense. But suppose by some kind of quantum entanglement that was not the case and in fact Riker’s mind was identical across both bodies: they thought and experienced the same things and can freely control each other’s bodies. Then we’d say there was only one Riker, who is bilocated. Though they are not numerically identical in respect to their bodies, their mind shares a causal history—and that causal history is numerically identical.

For an example of this, see Surrogates or indeed even The Matrix, where the causal history of the individual’s two bodies is not entirely identical (one body can sometimes be causally affected in a way not affecting the other), but their minds are identical enough to be numerically singular. There is no separate mind in the two bodies building different memories and making different decisions—unless and until there is, such as when a surrogate body is “stolen” and used by someone else, but then that is precisely why your using someone else’s body, once that is exposed as having happened, would not subject them to conviction for your crimes.

Contrariwise, in Being John Malkovich one person was able to enter and control another person’s body while the other person could only watch, helpless, inside that same body. Another scenario in fiction is in Angel, where a demon and a man shared the same body and could fight over who controlled it, just like in Venom, where it’s an alien and a man. Here we can tell the two persons are numerically different yet share some causal history (as their minds partially overlap in experience and can switch who is in control), but not so much as to destroy their numerically distinct selves, by dissolving them into a single self. (Whether “multiple personality disorder” is a real version of this or not is still debated, as well as whether it should get you an insanity defense. The answer is usually, at law, no, but by analogy to conjoined twins, where one committed a crime the other did not, yet for lack of any technology to separate them, both must serve the sentence.)

Which brings us back to the problem of teletransporter twinning. We have not yet faced a situation of “cognitive cloning,” so we have no vocabulary for that yet, and are inexperienced with it conceptually. We have biological cloning. And we have words and concepts for that. But no one thinks that therefore identical twins are identical persons, and the reason they are not said to be the same person is that they have separate causal histories (they are not numerically identical). They are not in each other’s minds; they operate independently of each other as soon as they exist as cognitive entities. They were once the same identical cell, so they share a common biological causal history, but that cell contained no cognitive apparatus. By the time their cognitive causal histories began, they were already biologically (hence physically) separate.

By contrast, a cognitive clone would be like a biological clone except, instead of splitting into separate cells from the same shared cell before becoming conscious beings, they split into separate persons from the same shared mind/brain—after already developing a cognitive history as a person. We can imagine an alien race that reproduces through budding, for example: they just divide into two at some point in their adult life, each one starting out with a copy of the same brain arrangement and thus being a singular person before the split, then being the same but separate people an instant after the split, and thereafter developing as separate people with independent causal histories (they are then not in each other’s minds; they operate independently of each other), just like Will and Tom Riker.

Those people would be the exact same person up to time t but (in the same connotation) different persons after time t. This would mean Will and Tom would both be guilty of a crime Will committed before Tom existed, but Tom would not be guilty of any crime Will committed after Tom split from Will (and vice versa).

Summary

All of these distinctions are possible to make because of ontological (physical) realities (numerical identity vs. numerical difference, and shared vs. separate causal histories; and the differences between mind and body, and hence between “person” and “chassis”), but are then made according to semantic choices (what kind of “difference” or “sameness” are we concerned to point out or reason from on any given occasion). And all of this is entailed on physicalism, and thus requires no further explanation. If physicalism is true, all of these types of identicality, sameness, and difference will necessarily exist (and always be verifiable in principle).

So the paradoxes of Sorites and Theseus do not exist because of any ontological ambiguities (except perhaps at the quantum level), but because of semantic ambiguities (such as the choice of what to assign the labels “mountain” and “hill” to, likewise “same” or “different”) and epistemic ambiguities (such as being unable to observe the ontological facts you have semantically chosen to matter). They therefore don’t problematize any ontology, physicalist or otherwise; but they do problematize all worldviews, and equally, since the same semantic and epistemic problems are faced in all. And yet all those problems have practical solutions, some of which I explored here.