How Many Earths?
How Many Earths?
Let’s start with a couple of thought experiments.[1] Suppose there is a certain planet that has been around for a billion years; call it Janet. Not much has changed in it during this time and it has received no bombardments from abroad. Then one day it is subjected to an intense heat wave caused by getting too close to a sun or by incoming meteors. This influx of heat causes Janet to undergo a thorough geological transformation: the rocks that compose it metamorphose into different kinds of rock. We feel inclined to say that Janet is not the same planet she was; indeed, Janet no longer exists, having been transformed into a different planet. The case is like transforming a person so comprehensively that we cannot speak of the same person. Janet now looks completely different, is half her previous size, and has a different molecular composition. She might even have been split into two. Janet is no more. Now consider another planet, Joan, that receives a different kind of treatment: Joan is subjected to heavy sustained bombardment from outer space, adding tremendously to its mass. It is now ten times bigger than it was and hosts types of matter alien to its original makeup. It isn’t itself internally modified, but it is covered with alien material. We would say that Joan still exists but the planet that has been created is not identical to Joan; it is a new planet. It is as if the new planet has swallowed the old planet whole. Here we would speak of two planets not one—old Joan and new Joan. Obviously, we can construct other scenarios of a similar kind; for example, we could imagine a planet, Julie, that is pulled apart by a strong gravitational field and now consists of spatially separate parts held together by a cosmic thread—it looks like three balls held together by string. Does Julie still exist? Judgments of planetary identity are not always straightforward; some changes can make us question a planet’s identity. We might even discover that a planet we thought was one is really two: under its surface are two previous planets that have tenuously joined forces, giving an appearance of unity that is not borne out by the facts (they have quite different geologies).
My question, then, is whether the earth is as unitary as we tend to suppose. Is it really one planet? What makes a planet into a single object? What are the criteria of identity associated with the sortal “planet”? Consider a planet very different from Earth called Wendy: Wendy is spherical like Earth but is made of only one type of rock, say granite; it is homogeneous all through. There is no molten core, no atmosphere, and no alien bombardment in its past. It is exactly as it was when it came into existence ten billion years ago—no life on it, no weather to speak of, no geological upheavals. Wendy is completely static. Nor does it vary from place to place—nothing like the arctic or the tropics. It is a very dull planet. But it is certainly unitary over time; it has not gone out of existence since its birth, replaced by a numerically distinct planet. Wendy hasn’t changed a bit. It’s the same old dependable Wendy over a lifetime of ten billion years. This is nothing like planet Earth, whose career has been notably dynamic: huge geological changes, much bombardment, considerable accretion, hard on the outside and soft in the center, continuously changing, and home to myriad life forms. Earth is barely recognizable from its early days; not the same planet at all. Wouldn’t it be proper to speak of Old Earth and New Earth? Isn’t that more respectful of the facts on the ground? Doesn’t it give a more accurate picture of what we loosely call “Earth”? Don’t we talk the way we do out of convenience not ontological veracity? It’s like the way we talk about towns: towns change dramatically over time, yet we speak of them as one. Isn’t the London of today a different town from the London of its first incarnation (a bunch of mud huts by a river)? Aren’t there really many Londons, over time and at a time? Don’t we speak of a single London only by convention? Certainly, being in roughly the same place is not sufficient for numerical identity.
I would therefore like to propose that we revise our thinking about the earth’s identity to take account of its actual nature. There are really several Earths. True, they are tightly crowded together, like cattle in a corral, but they are sufficiently distinct to warrant names of their own; the collection of them is crudely designated “Earth”. And I recommend going the whole hog: there are many Earths. There is crust Earth, mantle Earth, outer core Earth, inner core Earth, ocean Earth, atmosphere Earth. Crust earth can itself be divided into different Earths, according to the type of rock forming the rock strata. Then we have the different regions of surface Earth: arctic Earth, temperate Earth, tropical Earth. In addition, and importantly, we have botanical Earth, animal Earth, and psychological Earth.[2] Each of these is part of the totality we call “Earth”, though spatially separate. They each constitute different “worlds”: the world of molten lava, the solid crust world, the oceanic world, the plant world, etc. The main division is between the original world of early Earth consisting largely of hot molten rock and the cooler post-bombardment world of later Earth. The latter then divides into pre-biotic Earth and biotic Earth. We might then go on to distinguish the pre-mental Earth from the mental Earth. It would not be unreasonable to suppose that the original planet does not exist anymore, having been replaced by the newer model, the result of serious work being done on the original (like a thoroughly renovated house). That hot hell-hole no longer exists, being replaced by our very habitable semi-paradise. It looks and feels completely different. Its geological composition is completely different. It is no more the same planet than the original Earth was the same celestial object as the cloud of dust from which it condensed. Too much transformation, too much metamorphosis. From lava to life. What if planet Earth is transformed even more dramatically in the future, say by nanotechnology driven by AI—different geology, different climate, different everything, completely unrecognizable? There can be many Earths over time and many at a time. We live on a multitude of Earths, just as we live in a multitude of towns. Each should have its own name in an ontologically perfect language.
The earth is like the brain. What we call the brain is really composed of many sub-brains, and it is pretty arbitrary where we draw its boundaries (are the retinae part of the brain, or the whole nervous system?). We already have labels for the parts considered as separate modules; it is those that matter not the brain as a totality. The whole brain has no identifiable function; it’s the parts of it that have functions. It is easy to imagine a brain as a loose collection of separate entities linked causally; there is no necessity about cramming all the parts into a confined space. The various sub-brains could each have their own place of residence distributed all over the body (intellect in the toes, say). It’s the components that matter not the way they are packaged. Similarly, the earth is a package of sub-earths, each with its own identity, its boundaries unclear (does it include the clouds and atmosphere?). How many brains do I have? How many earths do I live on? Many, in both cases. Words like “brain” and “planet” are not well-behaved sortals, carving nature at its joints; they are folk terms introduced for convenience. They suggest more homogeneity than actually exists in their designations. We exist in a plethora of worlds; we live on a plurality of planets. If the molten core of Earth were to evaporate, leaving only the crust, we would still have a planet called Earth to call home—crust Earth, that is. If the crust were to disappear, leaving only the molten core, we would still have an Earth, though not one we could live on. I think, in fact, that throughout history the reference of “Earth” in people’s mouths was the part of Earth on which they lived—a small section of the earth’s surface. Most of what we now call Earth was outside their ken and conceptual repertoire; now we take in much more, but remember that many people know nothing of the full reality of the place in the universe they occupy. The folk never intended to take in the whole kit and caboodle, just a local slice of it. They were right in that there are many Earths; there is no single Earth, no natural unit, no primordial individual substance. There is no single thing that persists from Earth’s birth to now. There are many Earths whose fate we are (rightly) concerned about, not just one. When you gaze at one of those pictures of Earth seen from a distance, you are seeing many Earths not a single Earth. If Earth dies, many Earths die. If some Earths die, not all do.[3]
[1] I view this essay as a contribution to the philosophy of astronomy, including Earth science. It might help to consider my discussion of “planet” in conjunction with Quine’s discussion of “rabbit”, though rabbits are better defined.
[2] See my “A Philosophy of Nature” on the continuity of biotic and non-biotic Earth.
[3] We are told that Mars used to be a warm watery planet and is now a cold dry one. That planet is no more, having been replaced by a much less hospitable planet. What we see now is a dead body, a remnant. How many planets orbit the sun? We are accustomed to saying eight, but properly individuated there are many more. We alone have about twelve by my count. If we took all of our life forms, along with a suitable quantity of soil and rock, to another solar system and set up camp, we would still be living on Earth by my reckoning—an Earth anyway. If a cataclysm stripped Earth of all life, leaving only an inorganic hunk of rock, we would be left with an Earth of sorts, but not the one we know and love. We could call this the “Many Earths Hypothesis”, though it is not so much a hypothesis as a fact. The word “Earth”, without any attached modifier, suffers from indeterminacy of reference; context usually helps to narrow it down. It is high time we got more specific about what we are talking about.
Hmm. Is it true or false that Ockham lived in London? Or that the Romans founded London?
(Likewise of your “Mars used to be a warm watery planet”. Which planet are you referring to here? I think you are saying of Mars, the planet that is now cold and dry, that it it used to be watery. )
The sentence lacks truth-value pending a more definite definition of “London” (he didn’t live in the original settlement called “London”). The Romans didn’t found the city we call “London” today, but they did found an ancestor city. I am referring to a planet that is spatially continuous with modern Mars but not identical to it. It’s semantically like “I used to be a sperm and egg”.
“I am referring to a planet that is spatially continuous with modern Mars”
So it’s false to say that Mars used to be a warm watery planet? I would say definitely true, at least assuming that it (Mars) was warm and watery. The chatbot says “Yes, Mars used to have water”. The term ‘Mars’ here refers to Mars, that is, what you call modern Mars.
You are ignoring the point that the current planet called “Mars” is only dubiously identical to the watery planet that preceded it. That was the point of my example of Janet. Things can change enough over time that numerical identity fails to hold. Of course, we can keep using the same name for convenience, even when identity doesn’t hold. This is all elementary stuff that I shouldn’t need to explain. Being the same hunk of rock doesn’t entail being the same planet; compare statues and pieces of bronze.
This is by no means an elementary subject. Let’s go back to the statement of yours that began it for me. You wrote ‘We are told that Mars used to be a warm watery planet and is now a cold dry one.’
Looking at the sentence embedded in that ‘that’ clause, the grammatical subject of both ‘is now a cold dry one’ and ‘used to be a warm watery planet’ is ‘Mars’. If that sentence is true, it follows that there is a single object called ‘Mars’ that is now cold and dry, but was once cold and watery. Not two but one. Are you saying that what we are told is false? That perhaps Mars has always (as long as it has existed) been cold and dry, and never been warm and watery? The problem is that if you are right, many of the astronomical statements we hold to be true, such as ‘Mars used to be warm and watery’ are false. Perhaps also ‘The sun is 4.6 billion years old’.
Another problem is that directly after ‘We are told that Mars used to be a warm watery planet and is now a cold dry one,’ you write ‘*That planet* is no more …’. What does ‘that planet’ refer to? Presumably the planet that (as we are told) used to be a warm watery planet and is now a cold dry one, which implies that the planet that is now cold and dry no longer exists.
You go on ‘we can keep using the same name for convenience’ but how does that work for ‘Mars used to be a warm watery planet and is now a cold dry one’, where there is just one name serving as the subject for both predicates?
Reply to Buckner
I have already given you the materials with which to answer your questions, and these materials are commonplace in the philosophical literature on identity (see Wiggins, Ayers, et al). An oak tree has never been identical to the seed from which it sprang; rather, the tree developed from the seed and came into existence later than the seed. The tree has branches, the seed doesn’t. A bronze statue of a man was never identical to an earlier statue of a cat made from the same piece of bronze; rather, the two numerically distinct statues were made from the same piece of bronze. The man statue did not exist at the time of the cat statue, which went out of existence when melted down and made into a man statue. A person is never identical to the sperm and egg from which he or she came; rather, the person came into existence later as the sperm and egg gave rise to the person. I did not yet exist at the time a certain sperm and egg joined. A city like London did not exist at the time the first two mud huts that led to it were built (there is no identity between them); rather, the huts were the small dwelling which over time became a city. Mars was never identical to the cloud of dust that condensed into a piece of solid rock we call a planet; rather, the dust cloud condensed into and created that planet. No planets existed when the universe was just a vast cloud of dust, though the dust was the stuff that eventually constituted the planets. The planet we call Earth did not exist when the molten rock from which it originated came into existence; rather, that molten rock gradually turned into the planet we now call Earth. That rock still exists in solid form, but Earth is not identical to that rock—any more than a human being is identical to the bunch of atoms that composes his body. Nor is Mars identifiable with the planet that was once wet, watery, and home to primitive life; that planet is dead and gone, with only its cold dry corpse in its place. The Sun didn’t exist at the time of the big bang, but the materials that compose it did. Things come into existence and go out of existence all the time, though allied things pre-exist and post-exist them. We must distinguish origin and constitution from strict numerical identity. You can say “This man statue used be that cat statue”, but that is clearly false if taken literally as an identity statement. You can say “The Sun used to be a cloud of gas”, but that too is literally false if taken as an identity statement. There is no identity in these cases, though there are other close relations. You may wish to dispute some of these non-identity claims, and some of them are not trivial, but it is surely not absurd to insist on them, despite our loose linguistic practices. It is generally easy to paraphrase our loose statements into literally correct statements that don’t use the concept of identity. This is pretty elementary stuff for a well-trained analytical philosopher.
Mr. Buckner did reply to this long reply but I decided to delete it.
“The sentence lacks truth-value pending a more definite definition of “London” ”
We don’t need a definition of ‘London’. It’s this place where I live. As to the truth of ‘Ockham lived in London’, i.e. Ockham lived here, definitely true. He was at the Greyfriars studium near Newgate. Definitely doesn’t lack a truth value.
You couldn’t be more wrong. What about London, Ontario? The demonstrative “this place” could refer to any number of locations apart from London the city, even with a pointing gesture. It is not true that Ockham lived in the place called “London” well before his time; you need a time index at least. That early place is not numerically identical to the London he lived in.
Do I understand the Many Earths Hypothesis correctly as ontological, not epistemic? And if yes, why wouldn’t similar considerations be applicable to many other things, not just Earth?
Ontological, and yes they do apply to many things, such as towns, countries, buildings, brains, schools, and artifacts. Similar things have been said about persons, over time and at a time.
Referentially sloppy or imprecise discourse happens all the time; I notice it every day. (E.g., quantum physicists talking about “the collapse of the wave function”.) Wouldn’t insisting on “strictly speaking” discourse all the time be considered pedantic? (I’m not saying it’s not sometimes appropriate and necessary.) That’s why there is what is called “clarification”, something we need a lot more of from journalists in our political discourse. BTW, who first came up with the example of the ship that set out on a journey as the good ship “Jezebel” (the name of my favourite cat, if you’re interested) being damaged and then repaired on the high seas to the extent that when the work was done not a single piece of wood or cloth remained from the original ship, and posed the question, “Could the ship that continued on the journey still be called “the Jezebel”?”? If so, what is it that provides the continuity through the change? I may not have it right, but I’m sure you could provide the correct version.
I know it as Theseus’s ship. Doesn’t language have to be sloppy in order to work?