We normally identify the person today with the person yesterday, and with the person of years ago. Pete points to a picture from four or forty years ago and says, "There I am. That's me." Pete feels that something exists, his "I", which endures throughout his lifetime, although Pete's body today may differ greatly from the body in the picture. Someone asks Pete, "When were you born?" and Pete gives the age of his body, although his present body differs a lot from his birth body.

As a said entity, the "I" certainly exists. We undoubtedly have said identity and historical continuity with the person of years ago. But do we have a singular identity? Does "I" today refer to something in us perfectly continuous and changeless? Does the "I" today possess singular identity with any entity that existed a week ago, or even a second ago? Or does our "I" possess only said identity and historical continuity with some past entity?

In this chapter, we try to find the enduring "I". We explore these questions:

• In all the universe, only I am I. What makes me a unique individual, different from any other individual? What constitutes the foundation of my unique personal identity?
• Beneath changes in body, personality, memory and thought, what constitutes the unchanging kernel signified by "I"?
• What constitutes "I" in contrast to my possessions? As logically prior, the "I" must exist before it can have any possessions. Therefore, the "I" and its possessions do not intersect. We can have a possession but we cannot "be" a possession.
• In my life I have played roles such as schoolboy, friend, college student, spouse, professional, and writer. But just as we can ask of a play "Who is playing Macbeth tonight?" we may ask "Who or what is the 'I' behind my roles?"
• Something exists here and now that makes me, me. How can we describe that something?
• What constitutes my identity, my self?
• What makes me, me?

Answering one of the questions, it seems, would answer them all. So we refer to them in the singular, as "the question of personal identity".

We discuss some answers.

As we've shown, if our body constitutes our "I" then we have no unchanging identity, not even over a single lifetime. Such a view may seem antithetical to religions' views of us but at least one religion, Buddhism, has a similar view.

Buddhists recognize the "I" as a component entity consisting of body, sensation, feeling, thought and consciousness. Upon death, the components dissipate and the "I" ceases to exist. Thus, Buddhists have the doctrine of "annata" (of "no self" or "no soul) which says no enduring "I" exists which survives death, or that exists unchanging from moment to moment. The "I" of a moment ago and the "I" now, Buddhists say, resemble the relation between the candle flame of a moment ago to the flame now.

Because no enduring identity exists, Buddhists, strictly speaking, don't accept reincarnation. But they do accept rebirth. Just as the "I" of now possesses historical continuity with the "I" of a moment ago, they believe the "I" of the next life has some sort of continuity with the "I" of this life—likening the process to the candle of this life lighting the candle of the next, so that the flame possesses historical continuity.

Buddhists ultimately aim for "Nirvana", a state that extinguishes the candle, so that no further rebirths occur.

If we possess an enduring "I" but cannot ground it in matter, then some sort of nonmaterial foundation must exist. Rene Descartes, noted mathematician and philosopher, famously grounded the "I" in the mind.

Descartes arrived at his conclusion by initially setting out to rebuild philosophy on an undoubtable foundation; he began by doubting everything he could possibly doubt. Do Earth and sky exist for certain? No, said Descartes, for possibly an evil demon creates their appearance in my mind. And possibly the same demon creates the appearance of animals and other people. Possibly the demon creates the appearance of the entire external world. (Philosophers have since replaced Descartes' evil demon with a mad scientist who connects a "brain in a vat" to wires that stimulate the senses, creating the illusion of an exterior world. At least one popular movie uses the "brain in a vat" idea as a premise.)

Descartes could doubt the existence of the entire external world. What could he not doubt? Himself. "Cogito ergo sum", declared Descartes. Often rendered "I think, therefore I am" but better render as "I am thinking, therefore I must exist", Descartes' dictum describes the starting point of his philosophy: his own existence. Thus, according to Descartes our mind (which he considered roughly equivalent to our soul) constitutes our "I".

Open-ended question: "I am happy, therefore I am" or "I am in pain, therefore I am" seems as valid as "I think, therefore I am". Don't experiencing emotions, such as happiness, or physical sensations, such as pain, also prove I exist?

Open-ended question: Doesn't the first word of "I think therefore I am" assume the conclusion? Does not "I think" gratuitously assume the idea of an "I" who exists and thinks? Shouldn't we begin with "thinking exists, therefore something exists" or, even better, "consciousness of phenomena exists, therefore something exists?"

After some deliberation Descartes satisfied himself that the external world exists too and arrived at a dualistic view of the natural world where only two "substances" exist: mind and matter.

We briefly digress to discuss the idea of substance.

In Descartes' time, philosophers usually thought of substance as the bearer of properties. Substances exist independently while properties exist dependently. (Roughly, we may think of "substance" as corresponding to "noun" and "property" as corresponding to "adjective".) Thus, we can imagine a tall, tan, heavy lamp existing independently, but we cannot imagine accidental properties such as tall, tan, or heavy existing independently. Some thing, some substance—in this example, the lamp—must exist in which the tall, tan, heavy properties inhere.

Descartes regarded matter's essential property as extension in space, and mind's essential property as thought or thinking. So, he considered any material property, except occupying space, as an accidental property, and any mental property, except thought, as accidental.

Of course, extension in space implies the existence of space itself, but perhaps Descartes considered space an emptiness and not a substance. In contrast, the ancient Greek philosopher Democritus explicitly recognized the existence of space, which he called "the void". Democritus painted a materialistic view of the world where only atoms and the void in which they moved exists. About a century after Descartes, Immanuel Kant described space and time as "forms of intuition", thereby making them a function of the mind, of something the mind imposes on its perceptions rather than substances existing independent of the mind.

End of digression.

Many philosophers who accepted Descartes's dualistic mind/matter view of the natural world sought to justify and defend it. For instance, the famous philosopher Gottfried Leibniz, who with Newton discovered calculus, tried to prove we cannot regard mind as a property of matter. In his mill argument, Leibniz asks us to imagine the mind as a huge mill of such size that we can enter it and look around. What will we see? Gears and levers and other material entities; but nowhere, said Leibniz, will we see a thought, nowhere will we find a gear or lever that thinks. Thus, concluded Leibniz, we cannot regard mind as a property or attribute of matter. Therefore, mind must exist independent of matter, as an independent substance.

But philosophers wondered how two independent substances like mind and body could communicate. For instance, if I decide to rise my arm, how does my mental decision affect my material body and cause my arm to raise? Or if I step on a pin, how does my body communicate pain to my mind? Indeed, why should my body communicate pain to my mind at all if the two exist as independent substances? Philosophers labeled such problematic questions the "mind-body problem". Defenders of Descartes offered answers, sometimes farfetched answers. For instance, Nicolas Malebranche, a French priest, claimed mind and body could not communicate but that when the thought arises in our mind, God takes notice and causes our arm to rise. Similarly, when we step on a pin God causes our mind to feel pain.

Today, researchers don't define "mind" as a substance that has the essential property of thought. We may describe contemporary usage of "mind" as that which:

• receives the reports of sense data or sensations, caused by the five senses reacting with the external world
• creates its own sensations when dreaming or hallucinating
• forms perceptions (Sensations and perceptions differ. To illustrate, in an optical illusion sensations correspond to external reality, but what the mind makes of those sensations, that is, the perceptions it creates, may not.)
• experiences emotions such as happiness
• forms thoughts and beliefs
• stores and retrieves memories
• comprises our personality: our temperament plus our interests and talents. (The English word personality suggests this mix comprises the foundation of personal identity.)

A great deal of evidence indicates an intimate connection between "immaterial" mind and material brain, so much so that many researchers reject the idea of immateriality and describe the mind as simply "what the brain does". (Notice that this view of mind implicitly denies Descartes' two substance, mind-matter dualism, and resolves his mind-body problem by making mind a function of matter. Notice, too, that "what the brain does" corresponds to our idea of an act: that is, we might say "the mind is an act of the brain" just as "a fist is an act of the hand." Thus, we might regard mind as an emergent property of matter.)

We review four types of evidence for an intimate mind/brain connection, evidence for the view that "the mind is what the brain does":

• 1. the effect of intoxicants and medications on the mind
• 2. modern passive imaging techniques (e.g., EEG, MRI, fMRI, PET, NIRS, MEG) that show what parts of the brain "light up" during certain functions
• 3. active electrode stimulation of parts of the brain
• 4. behavioral changes when accident, disease or surgery damage a portion of the brain

1. We need not describe the well-known effects of caffeine, nicotine, alcohol, marijuana, cocaine, LSD and heroin on the mind, effects which demonstrate an intimate link between body chemistry and mind. In recent decades antidepressant medications have transformed the personalities of many people, implying personality does not constitute the unchanging foundation of personal identity.

2. As an example of imaging techniques, PET scans show that violent criminals have less frontal brain activity than normal (frontal activity dampens emotions such as rage so less frontal activity implies an inability to suppress rage).

3. As to electrodes, stimulation of the temporal lobes provokes vivid recall of long-forgotten songs or childhood memories; stimulating one part of the amygdala creates feelings of fear and panic, while stimulating another part creates warm, friendly feelings. Stimulation of the temporal/limbic system may produce intense feelings of joy and even a sense of God's presence.

4. As to brain damage:

• after surgery removed a tumor and some surrounding brain tissue a man lost his ability to feel emotion
• after a brain injury a farmer lost the ability to recognize faces; a man with a similar condition once passed his mother on the street and didn't recognize her. A 66-year-old woman had the opposite problem: she mistook strangers as her ex-lover and his girlfriend in disguise, trailing her; a CAT scan showed a stroke had damaged her cerebral cortex
• a woman with a damaged hippocampus could not remember anyone for more than a few seconds so her physician would reintroduce himself several times each visit
• a frontal lobe tumor apparently triggered obsessive, abnormal sexual interests in a 40-year-old man, who returned to normal once the tumor was removed. When the interests later returned, his doctor discovered the tumor had regrown
• in 1966, a churchgoing, ex-Marine, charity worker climbed a university bell town and over the next 96 minutes randomly killed 13 people and wounded 30 others. An autopsy found a walnut-sized tumor pressing on his amygdala causing his amygdala "to fire in a way that would normally only occur in situations of great danger, threat or challenge"

Such evidence supports the view that "mind is simply what the brain does". If we accept that mind equals "what the brain does" equals "I", then we get a version of the natural answer which says I cease to exist at death, when my brain ceases to function.

Can we find for our theology some enduring foundation for our personal identity? Or can we find no answer but the natural answer?

On first sight, the prospect doesn't appear promising. Today, science's worldview acknowledges four fundamental forces: gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. None of them in any obvious way can support personal identity. Further, the universe appears "causally complete"—every physical effect has a physical cause if it has a cause. (We say "if it has a cause" because quantum mechanics describes some behaviors with merely statistical, not deterministic, laws. For example, it describes the decay of radioactive atoms statistically, in terms of half-life, but does not specify whether a cause exists for why this atom decays rather than another. Some interpretations of quantum mechanics say no immediate cause exists for the decay of a particular atom; others interpretations, ex., Bohmian Mechanics, theorize an underlying cause.)

As an illustration of causal completeness, imagine I decide to drink some water. What causes my arm to grab the glass? Contraction of the muscles. And what causes muscle contraction? Electrical impulses from my brain. And what causes the electrical impulses? Motor neurons firing, caused by . . . caused by . . . As far as we can determine, a physical cause always precedes the physical effect. Nowhere in the chain of causes do we find consciousness or immaterial mind, nowhere do we find my mental desire to drink water as a cause of anything physical like my arm moving.

The uninitiated reader may find that unbelievable, so let's say it again: no currently-known scientific law can account for how a conscious immaterial thought or desire to drink causes my body to take a sip of water. Causal completeness does not allow consciousness or thought to impact the physical world. (On occasion, news stories appear such as "paralyzed patient moves prosthetic arm with thoughts alone". In actuality, the prosthetic arm senses electrical impulses in the brain, but no one understands how the patient's thoughts can trigger electrical impulses.)

Our experience seems to contradict causal completeness: from my point of view, my conscious thought or desire to drink initiates the chain of cause and effect that ends with my drinking. But how can a conscious thought initiate a chain of physical causes and effects? No known law of physics allows my thought itself (as opposed to my brain's electrical signals) to move one atom, much less cause macroscopic changes in my body such as arm movement.

In the 19^th century, many scientists considered science's knowledge of the world essentially complete. Mechanics, thermodynamics and electromagnetism explained almost all know phenomena. (Thus, the story that in 1874, a physicist professor advised 16-year-old Max Planck not to study physics because "in this field, almost everything is already discovered, and all that remains is to fill a few holes".) Filling "holes" such as the ultraviolet catastrophe, the photoelectric effect, and the Compton Effect, led to quantum mechanics and relativity and revolutionized our understanding of the natural world.

Today, consciousness seems to constitute a hole in science's understanding of the natural world, a mystery many philosophers and scientists acknowledge and work to solve. We'll call this mystery of how our conscious thoughts and desires interact with the body, the consciousness-body problem. (It bears an obvious similarity to Descartes' mind-body problem.)

The consciousness-body problem suggests a deeper problem still: how can consciousness itself exist? How can unconscious matter ground consciousness?

Descartes begins with the mind, so the problem of how the mind can exist does not arise. But if we update Leibniz's argument, we might imagine shrinking ourselves to microscopic size and witnessing what occurs inside a human body. We would see muscle contractions and chemical reactions and electrical signals and neurons and synapses, but we'd never see consciousness or emotions or thoughts. Or going further, we'd see atoms, or protons and electrons, or quarks; or gravity, electromagnetism, the weak nuclear force, and the strong nuclear force—none of which, in our current understanding, can support or ground consciousness.

So we arrive at our "fundamental question of consciousness": does consciousness somehow emerge from matter, or should we consider it a separate element, similar to Descartes' dualistic view of mind as a separate substance?

To dramatize the fundamental question of consciousness and make its issues concrete, philosophers ask, "Can a philosophical zombie exist?" Unlike the mindless, undead zombies of fiction, the philosophical zombie (or simply "zombie") duplicates atom-for-atom a normal person but lacks the inner experience of mind and consciousness.

For instance, imagine an atom-for-atom replica of Aunt Sally, a zombie Aunt Sally, who behaves normally in every way but who lacks inner experience. Can such a being exist? If matter grounds consciousness, then no, because atom-for-atom matching would imply all material properties match, too. But if consciousness constitutes a separate entity (Descartes would have said "a separate substance"), then yes, a zombie might exist.

We can think of zombie Aunt Sally as an extremely capable and sophisticated robot, constructed not of computer chips and motors, but of flesh and blood, who has the consciousness of a robot—that is, none. Although she (or "it"?) may seem bizarre, the zombie Aunt Sally would violate no known scientific law. Zombie Aunt Sally appears consistent with science's worldview because that worldview contains no hint that consciousness exists. We have the idea of consciousness only because we have it ourselves. (One interpretation of quantum mechanics does speculate that consciousness "collapses the wave function" but the interpretation doesn't answer how consciousness arises.)

If consciousness does indeed constitute an entity independent of matter, separate and outside known scientific laws and principles, then conceivably it could continue to exist once our material body devolves and fades back into its elements. When we finally understand consciousness we might find in it a foundation of personal identity which survives death.

Does any evidence exist for consciousness existing outside a causally complete universe, independent of matter? Some people claim evidence such as:

• people sometimes have an impaired or even missing portion of their brain (i.e., grave hydrocephalus, where an abnormal quantity of cerebrospinal fluid replaces brain tissue), yet function normally
• children are sometimes born with amazing and inexplicable knowledge of a past life, supporting the idea something survives death and can reincarnate
• hospital patients sometimes fall into a near-death state and, when revived, accurately report what occurred when clinically unconsciousness, which support the idea consciousness can sometimes exist independent of the body

Much work lies ahead before we understand consciousness as thoroughly as we understand, for example, electromagnetism. Yet, we understand consciousness well enough to recognize various problems with accepting it as the foundation of our personal identity:

• our consciousness waxes, wanes, and, in dreamless sleep, seems to leave us entirely. Indeed, a few days of continuous forced consciousness (sleep deprivation) threatens our mental health and even our life. So how could something we cannot bear continuously for more than a few days constitute our real self?
• if we agree, "consciousness is what the brain does", we liken it to an act. But an act cannot possess a singular identity.
• or we might model consciousness as an emergent property of the brain, in which case it could not survive the death of the brain.
• even if we think of consciousness as existing independent of the material universe, it seems like a light illuminating the "room" which contains our mind, specifically, our sensations, perceptions, emotions, thoughts, beliefs, memories, personality. Thus, we might reasonably identify our personal identity with the mix of entities that constitute our mind, rather than the impersonal light illuminating it.

Undoubtedly, we have much to learn about consciousness. But given our present state of knowledge, we cannot confidently answer the question of personal identity with consciousness.

Because we accept science's ontology, our theology can't include a supernatural soul. But let's for a moment assume soul exists and ask, "Could soul answer the question of personal identity?" Some thoughts.

First, we need an idea of what "soul" means. Although we hear phrases such as "my soul" and "your soul", speaking of soul as a possession doesn't make sense. A possession may suffer some unfortunate fate which leaves me unharmed, as when a fire damages my home. At the moment, a fire may be burning a shirt in my closet, and I wouldn't know it or feel any pain. Theists may sometimes speak of soul as a possession, but they don't think of it as such. Their concern about the eternal fate of "their soul" demonstrates they consider the soul not a mere possession, but some or all of "what they really are".

For a definition of soul we'll use "the immaterial, spiritual, immortal entity that comprises our enduring 'I'". But this definition puts us on the horns of a dilemma. The question of personal identity asks what makes me, me, as I exist here, in time and space, now and in the future. But if we take soul as immaterial and spiritual, as completely outside the universe of space and time, then it cannot comprise any part of what I am here, now, at this moment. On the other hand, if soul somehow comprises part of what I am here and now, then at least some portion of soul must exist in the here and now, in space/time—that is, soul and what I am as a material being must intersect. But this contradicts the "immaterial, spiritual" part of our definition of soul. Thus, it appears soul as we've defined it cannot answer the question of personal identity.

A theist might easily dispute our brief analysis of soul and personal identity. First, perhaps a different definition of soul would lead to different conclusions. Second, perhaps the supernatural in some mysterious way penetrates or upholds the natural, which would allow soul to manifest in the here and now. Third, although our body exists in space/time, the theist could say the "real me" does not, but it exists as an entirely spiritual being—a soul.

So let's grant the theists' conclusions and assume that soul "is what we really are". Let's assume that I am a spiritual, immortal soul, destined to spend all eternity with God; let's assume that soul constitutes my singular identity, my unchanging essence, which exists forever. And let's examine consequences of those assumptions.

We change throughout our lives. Our bodies, personalities, beliefs, attitudes, talents, abilities all change. We acquire virtues and faults and, sometimes, lose them. Which of the various "I"s that I have been throughout my life exist in my soul? Which of the various "I"s go to heaven, and which eventually cease to exist?

To illustrate, let's imagine Aunt Sally lives a checkered life and passes away at age ninety. We imagine her as a sweet, innocent schoolgirl, a vivacious and vigorous twenty-something, a caring, responsible mother, and a loving grandmother who indulges her grandchildren.

What parts of Aunt Sally's personality inhere in her soul and accompany her to heaven? Which do not? Let's assume Aunt Sally has some characteristic faults, as everyone does. Suppose she gossips, envies, and sometimes acts unkind. We may suppose she often feels hate for some people, or has an addiction. Do these faults vanish at heaven's gate? If so, can we identify the Aunt Sally who lived on Earth with the sanitized version of Aunt Sally who lives in heaven? If half of Aunt Sally's personality characteristics vanish in heaven, then doesn't a new person come into existence in heaven? If so, hasn't the Aunt Sally who lived on Earth ceased to exist? If so, isn't the promise of eternal life for the Aunt Sally who lived on Earth unfulfilled?

We can ask a similar question: supposing we get a body in heaven, which body does Aunt Sally get? Her schoolgirl body? Her twenty-something body? Her ninety-year-old body?

Another question: after Aunt Sally reaches heaven, can she still change and grow?

If we answer "no", then we condemn Aunt Sally to an eternity of existing as a sanitized, but limited being, with the limited interests and knowledge she acquired on Earth. But if she exists as a limited, finite being for all eternity, why would we call that heaven? (Imagine the child who dies at age two. Will the child through all eternity remain a child in personality, emotional maturity, and knowledge? That hardly seems a desirable fate.)

If we answer "yes", then Aunt Sally in heaven can change and grow. Can she learn quantum mechanics or analytic philosophy if she wishes?

Let's suppose Sally can change and grow. Now imagine that when she passes at ninety, she leaves a great-granddaughter, Nancy, of ten. Nancy also lives to ninety and for eighty years looks forward to meeting Aunt Sally again in heaven. But when Nancy arrives, does she meet the sweet, old great-grandmother she remembers? Of does she meet someone who has the vivacious body of a twenty-something, who lacks any of Aunt Sally's characteristic faults, and who understands algebraic topology?

Again we meet the problem of personal identity: can we identify the being Nancy meets in heaven with the Aunt Sally who lived on Earth? Or has the Aunt Sally of Earth in actuality ceased to exist?

Moreover, suppose Aunt Sally can grow in knowledge, wisdom and power. Can she eventually grow to rival the highest angels? If so, it appears eventually she would hardly differ from God. As such, the heavenly Aunt Sally would possess historical continuity with the earthly Aunt Sally, but singular identity would not exist. The earthly Aunt Sally would no longer exist. Therefore, the salvation offered by many religions fails to achieve its goal: eternal life for the "I" which exists here and now, on Earth.

Eternal life allays fear of death and annihilation; at first sight, it seems wonderful and reassuring. But it implies we must endure some sort of limited, less-than-God existence, for all eternity. Of course, many people would gladly give up their personal identity now for the opportunity to evolve over time into a godlike creature. But on reflection even a theist might wonder, "Do I really want to be me forever, eternally me, eternally distinct and different from God?"

§

It appears we can find nothing but the natural answer in our theology. Notice, the natural answer accords with our fundamental assumption of monism, because if we admit a single, ultimate ground of existence then my "I" must cease to exist somewhere down the ontological chain. That is, a table must cease to exist as a table as we descend the ontological chain to molecules, to atoms, to quarks, to the One. Similarly, "I" must cease as me as we descend the ontological chain, because at the ultimate level only the One exists. Thus, monism precludes the existence of an eternally existing "I", an eternal soul, separate from God.

So even if someday we find something in us that survives death, monism rules out it surviving forever. For instance, suppose one day we understand consciousness and discover it can somehow survive the death of the brain and body. Suppose we verify that children sometimes recall past lives because, in fact, they lived them. Then we'd have to modify our view of personal identity, but not fundamentally. Whether the "I" ceases to exist after one life or after a hundred, it must (according to our view) eventually cease to exist because on the ultimate level only the One exists.