"What is God?" and "What happens after I die?" surely rank among important questions thought unapproachable by science. We've already addressed the first question by defining God within our theology. (Later, we address the related question, "Does God as we've defined it really exist?") But a mere definition cannot answer "What happens after I die?"

Can we answer that question by drawing on material we've already discussed? For instance, science's ontology includes the natural world but does not include the supernatural. So, if we rule out souls and reincarnation, it appears only one possible answer exists: the "natural answer"; that is, I cease to exist at death, when my body ceases to function.

What more can we say? Much. First, before we can discuss its ultimate fate we need a clear idea of what constitutes the "I". Yet notice the natural answer identifies the "I" with the body and/or the body's functioning:
      I cease to exist at death, when my body ceases to function.
But it also speaks of the body as a possession:
      I cease to exist at death, when my body ceases to function.
"Body" may constitute me, or may constitute one of my possessions—but not both. (Notice, we have the same problem if we answer, "I cease to exist at death, when my brain ceases to function".) Clearly, we need a better idea of what constitutes the "I"—of what constitutes our identity, our self—before speculating about its fate.

We postpone discussion of answers other than the natural answer until the next chapter. In this chapter we explore the idea of identity in general. We discuss three types of identity: other, singular, and said. Then we revisit the natural answer in light of what we've learned. In the next chapter we examine other answers to the question "What happens after I die?"

We begin by informally introducing some ideas about identity in a dialogue.
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A: As a child I had a pet cat called "Snowball". Where is Snowball now?
B: He's dead and gone, unfortunately.
A: "Dead" isn't a place; "gone" says he's not here. I know he's not here, but my question is "Where is Snowball now?"
B: He is nowhere; he has simply ceased to be.
A: Acts can simply cease to be. When I stop singing the act of singing doesn't go anywhere: it simply ceases to be. But Snowball had a physical body. In fact, unless animals have eternal souls, Snowball was a physical body. Conservations laws apply to physical bodies. Once we thought there were two conservation laws—"matter can neither be created or destroyed" and "energy can neither be created nor destroyed"—but eventually we learned matter may be converted into energy and vice versa. So now we know there is only one conservation law: "matter/energy can neither be created nor destroyed". So Snowball must still exist, in some form or other.
B: Well, yes, the atoms that composed Snowball's body still exist, certainly. When Snowball died his body's atoms eventually returned to Earth's biosphere. Some of Snowball's atoms are in the ocean; others in the atmosphere; others in people and things we see around us.
A: So Snowball is still here, all around us?
B: Not really. Even while Snowball lived atoms continuously entered his body and eventually left. So we shouldn't say Snowball was a physical body. That is, we shouldn't identify Snowball with any set of atoms. Rather, Snowball was an act.
A: I don't understand.
B: It's as if Snowball was a whirlpool. In a whirlpool the water continuously enters and leaves but the act, the flow—the spinning of the water—persists and is what we call a whirlpool. When the flow stops, when the act of turning stops, the whirlpool simply ceases to exist. Similarly, when Snowball died the "flow" that was Snowball simply ceased to exist. True, the particular atoms that comprised Snowball at that moment still existed. But when Snowball lived, the atoms weren't Snowball; rather the flow was. So when the flow stopped, Snowball died. And the particular atoms that comprised Snowball at the moment of death still existed, and still exist, although the Earth has reclaimed them and now they form parts of other flows, of other people and things.
A: So Snowball is really gone? Snowball has just ceased to exist?
B: Yes. Snowball was an act, a continuous flow, a whirlpool of atoms, and when the flow ceased to exist, Snowball ceased to exist—unless he had some sort of eternal soul that now resides in some cat heaven.

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We now begin our investigation of identity in general, postponing discussion of answers other than the natural answer until the next chapter.

"Other identity" denotes the identity of two or more distinct entities, for example, when we say, "Take any seat, they're all the same." Of course, two distinct entities always differ in some way or we wouldn't call them distinct. We use the term "other identical" when we judge any differences inconsequential. Thus, the foundation of other identity consists of relevant common properties (i.e, common properties that we judge relevant).

As an example, consider two coins differing only in that one lays face up and the other, face down. Most people would judge that other identity holds for two coins. Or consider electrons and atoms. Scientists judge other identity holds for electrons and atoms when they speak of the electron and the hydrogen atom. That is, although atoms may differ by isotope and electrons may differ by spin, they judge that nothing fundamentally differentiates one hydrogen atom from another, or one electron from another. (In effect, they judge isotope and spin as accidental properties; we discuss accidental properties below.)

Of course, what one person judges as inconsequential, another may not. So other identity involves judgment. For example, a man says, "Your socks don't match." A comedian responds, "They do; they're both cotton."

As another example, imagine a machine that creates a perfect atom-for-atom duplicate. We duplicate a table and judge the two tables as other identical. Then we duplicate Leonardo da Vinci's famous painting, the Mona Lisa. (Or, rather, we duplicate the painting as it exists today, a descendant of da Vinci's original Mona Lisa. "Descendent" because today's Mona Lisa differs from what da Vinci painted: pigments have aged, colors have faded, etc. We say the descendent has "historical continuity" with what da Vinci painted.) Yet although the duplicate matches atom-for-atom, a reasonable person might not judge them as other identical. Rather, a person might judge the duplicate as different from and inferior to the descendent. An art collector would probably value the descendant more than the perfect atom-for-atom duplicate, because the collector regards historical continuity as part of a painting's identity.

Thus, when we judge the two tables other identical we implicitly judge that relevant properties do not include historical continuity. But for the painting, we implicitly judge that relevant properties do include historical continuity.

"Singular identity" indicates the sameness or changelessness of a single entity over time, as when we say "I want the same seat I had yesterday." For other identity, we may or may not include continuity as one of the relevant properties. But for singular identity, some type of continuity seems integral and implicit.

For a nonphysical entity, such as a concept, continuity means an unchanging definition of the concept. For a physical entity, such as a seat, continuity means the same "stuff", i.e., that the same matter constitutes the seat today as yesterday.

(Singular identity seems to capture the idea of personal identity; of an enduring, personal "I'; of a constant, unchanging kernel that exists now, identical to what existed at birth, when my body had a small fraction of its present mass, when I had a vastly different personality, different emotions, different memories, different mind.)

Can anything satisfy the definition of singular identity? That is, can we name an entity that possesses perfect changelessness, perfect continuity? Some concepts appear changeless. For instance, mathematical facts like the Pythagorean Theorem or relations like "more than" seem to continuously exist unchanged (although some philosophers use "subsists" to describe concepts and reserve "exists" for entities in space/time). Thus, some concepts may possess singular identity.

Can any physical entity possess singular identity?

A component entity will satisfy singular identity as long as its components do so and the relation between components remains unchanged. For example, a table will satisfy the definition if:
Thus, we see a component entity's singular identity dependents upon its components.

When a physical entity matches atom-for-atom now with an earlier version of itself, we say it possesses "continuity of matter". We may question if continuity of matter ever obtains on Earth for any length of time. For instance, an atom-for-atom match for the table will fail if a stray carbon-14 atom in the table spontaneously decays. In metals free electrons migrate from atom to atom, destroying an atom-for-atom match. Moreover, the sun shoots billions of neutrinos through each cubic centimeter of Earth each second, so even when we have an atom-for-atom match we may still judge that overall the "stuff" now doesn't match before, so that singular identity does not obtain. But we might suppose elementary particles, like quarks, possess singular identity because (as far as we know today) they possess no components.

What about the One? Does it possess singular identity? If we regard it as "that which acts" then some sort of change seems implied. Yet, on the plane at which it exists, nothing else exists. (That is, if we assume monism, then at the ultimate level only the One exists.) Thus, the One cannot differ from itself. So we may judge that the One does possess singular identity regardless of what it does, just as we consider a person the same person regardless of what they do. As we recognize an actor playing a role still himself, we recognize the One as still itself, regardless of what it does.

If we recognize the One as the constant, unchanging kernel existing now identical to what existed at the birth of our present universe, we may call it the "self of the universe" or simply the "Self" (with a capital "S"). So we regard "the One" and "the Self" as synonymous.

A final point: suppose electron A created now differs not at all from electron B created ten years ago. That is, suppose other identity applies between electron A and B. Then we might suppose electron A of ten years hence would not differ from electron A as it exists now. So we could conclude electron A possesses singular identity with itself. Thus, the other identity of electron A and B implies the singular identity of electron A with itself.

Before discussing the last of our three types of identity, said identity, we explore a framework used by philosophers over the centuries to analyze questions of identity: essential and accidental properties.

Philosophers long ago developed a workable definition of identity based on a distinction between essential and accidental properties. They call a property an "essential property" if that property makes the thing what "it is". More precisely, if an entity in all its possible states of existence has a property then we call that property an essential property. On the other hand, philosophers call nonessential properties—properties an entity can gain or lose and still remain the "same" entity—"accidental" properties. Thus, as long as something retains all its essential properties, it retains its identity.

To illustrate, the number of protons constitutes an atom's essential property; thus, a carbon atom in all its possible states of existence has six protons. A carbon atom cannot gain or lose a proton without losing its identity and becoming a different element (nitrogen or boron). On the other hand, the number of neutrons or electrons constitutes accidental properties, so ions and isotopes preserve an element's identity.

As another illustration, the ability to provide light constitutes a lamp's essential property; the lamp's color, height, and weight constitute accidental properties. If a lamp can't light then it can't function as a lamp, but we can change its height and it remains a lamp. A home's essential properties include the ability to provide shelter; its accidental properties include its exterior wetness and color. So we recognize a home after a rainstorm or one we've painted as the "same" home.

The essential/accidental definition of identity allows us to regard a thing that has changed as the same thing, but it seems to presume that essential and accidental properties exists in an ontological sense, i.e., in reality, independent of our judgment. To us, a home's essential properties include the ability to provide shelter, but termites might consider the ability to provide food an essential property. As another example, if we use a lamp as a paperweight then we no longer regard it as a lamp, and no longer regard the ability to provide light as an essential property.

We make a property essential or accidental by judging it so. Thus, essential and accidental properties do not inhere in the entity; rather, someone judges which properties "make the thing what it is" and which properties can change. So instead of using an essential/accidental definition of identity that seems to assume an implicit judgment, in our theory of identity we have judgment play an explicit role.

Only a few entities—concepts; possibly elementary particles and the One; probably not any complex physical entity for any length of time—satisfy singular identity. Yet we routinely identify things today as the same things we saw yesterday, although people and buildings we see today have changed, perhaps imperceptibly, and therefore do not satisfy the definition of singular identity. Our concept of singular identity appears too strict for most uses. Can we define a more useful type of identity? Might relaxing the requirements of changelessness and perfect continuity yield a more reasonable concept? We might try allowing historical continuity; we might allow tiny changes yet still regard the entity as the same entity, in some sense.

Allowing tiny changes often seems sensible. For instance, after a single uranium atom decays we may sensibly call the table the same table. And a concept may change, often as a response to better knowledge, yet we consider it the same concept. For example, astronomers once defined "planet" as a heavenly body which revolves around Earth. Later, they realized planets revolve around the sun and refined "planet". Eventually, they refined "planet" yet again so as not to include Pluto. Thus, when astronomers discuss planets today, they do not speak about the same thing as before; thus, singular identity does not obtain between present and earlier meanings of "planet". But if we allow historical continuity (instead of perfect continuity) then astronomers do speak about the same thing. (However, we must use caution because logical fallacies such as "no true Scotsman" or "moving the goalposts" rely on shifting definitions.)

Notice, historical continuity involves an element of judgment, specifically our judgment if a lack of perfect continuity negates identity or not. It also involves a judgment as to whether a change qualifies as tiny or not. We'll find it convenient to drop the "tiny" qualification. Therefore, we'll call two distinct entities, or a single entity at two different times, "said" identical when we judge any dissimilarities or discontinuities as inconsequential (that is, when we judge sufficient similarity and continuity exists before and after the change to regard the entity as the same entity). So, two entities qualify as said identity simply when we judge them as said identical, despite any ontological differences.

Said identity seems to capture the ideas of "identity" and "sameness" as commonly used.

(Notice that because other identity involves judgment, we may regard it as a variant of said identity. Singular identity involves judgment, too, but in different way. With other and said identity, we agree ontological differences exist but must judge whether the differences matter or not. With singular identity any ontological differences matter if they exist, but we must judge if differences exist or not.)

Let's discuss said identity in relation to acts, motions and flows.

For acts or motions, we must judge if/when a break in continuity invalidates said identity, where continuity may mean continuity of matter or historical continuity.

As to continuity of matter, if person Y recognized the ship of Theseus as an act (or component entity), then they might judge the original and the updated ship not said identical because the updated ship lacks continuity of matter with the original ship. They might argue that when a component of a component entity changes, the entity changes and so cannot be called identical. Further, if they save the original, discarded timbers of the ship of Theseus and after two decades use them to construct another ship, they might say continuity of matter (i.e., using the original components) makes it the same as the original ship.

As to historical continuity, if I make a fist with my hand, open my hand, and remake the fist an hour later (and, assuming for the sake of argument, my hand's atoms don't change in the meantime), have I made the same fist? The answer depends on our judgment. Similarity, when we regard a table as an act (the act or motion of components maintaining the same relation to each other through time), then if we disassemble the table and later reassemble it in the exactly same way (and again assuming identical atoms) we must judge if the reassembled table possesses said identity with the earlier table or not.

Because flows never have continuity of matter, we must judge only if/when a break in a flow's historical continuity invalidates said identity. To illustrate, if person Z recognized the ship of Theseus as a flow, where new components regularly replace old components, they would regard the updated ship as said identical with the original. (If person Z recognizes the ship as a component entity but allows historical continuity then they would also regard the updated and original ship as said identical.)

As long as a flow like a flame continuously burns or a whirlpool continuously turns, we may judge them said identical with themselves. But what about breaks in continuity? For instance, if we extinguish the candle flame now and relight the candle in a minute, do we have the same candle flame? What about a day later? Or two years? If we stop a whirlpool today and restart it tomorrow, can we say we have the same whirlpool? Can we judge a restarted version as identical to the original version? If we wish. Or we might decide the break in continuity makes the restarted version a different entity.

We have a similar choice concerning the claim that the University of Cambridge has existed since 1209. Recall, we picture the University as a flow, an educational process. But the flow stopped for two years when the plague of 1665 closed the University. (During that period, Isaac Newton did some of his most outstanding work.) So, like the restarted whirlpool and candle flame, we can judge today's University sufficiently continuous with that of 1209 and call it said identical—or not.

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Let's now revisit the natural answer, which identifies the "I" with the body. Does identifying the "I" with the body make sense? Not to theists, who regard their soul (Christian) or atma (Hindu) as their true self. But the nontheist often does identify "I" and body. But if I identify "I" and body, and if my search for an "I" means a search for my singular identity, then either 1) matter, specifically atoms, somehow comprise my "I", or 2) no "I" truly exists.

Certainly, atoms—mostly hydrogen, oxygen, carbon and nitrogen, as we've seen—comprise my body. But where did those atoms come from? All except hydrogen come from the belly of a supernova. And how old are those atoms? Perhaps billions of years. But how can atoms existing from long before my birth somehow become me for a while, cease to be me when they leave my body (and become me again if they return)?

But atoms do leave and enter my body, continuously. Our body changes with each breath, and over seven years replaces each atom; so even if my body now has some atoms it had at birth, it hasn't possessed them throughout. So how can my singular identity rest on the continuous flow of atoms that comprises my body? Evidently, it cannot.

What type of identity can a flow like a whirlpool or the human body support? As the candle flame now descends from the flame of the past, our body now descends from our body of the past—that is, it possesses historical continuity with our earlier body. But over time, what can we find that persists in the body, that we can point to and say "That comprises our enduring, unchanging singular identity"? Nothing material.

Regarded as a material entity, we possess only said identity—we possess identity merely because people commonly say we have one. So, if we accept the natural answer then we possess no singular identity, no true "I", even from moment to moment, much less over a lifetime.

Yet, we don't feel we lack identity. Rather, we feel at some level we are the same person we were when we were born. Certainly, our body changes, our personality changes, in fact, almost everything about us changes, but we feel that, somehow, underneath it all an unchanging kernel that is "I" persists.

But if our body does not constitute the foundation of our "I", if the foundation of our personal identity, then where might that foundation reside?