1. The thread connecting past states of matter to future states of matter is the existence of natural laws. Without these, there would be no determinism, because even with infinite knowledge of the present, it would be impossible to predict the future. While some people view the reliability of natural laws as a sign that free will does not exist, Smullyan argues that the laws of nature “are merely a description of how you act, not a prescription of how you should act, not a power or force which compels or determines your acts.” To use an example that is more concrete than human behavior, planets do not orbit the sun because of the law of gravitation. Instead, the law of gravitation exists because it is a reliable way to predict what the movement of planets around the sun will be. The same holds true for humans. A simple law of human behavior is that people make choices that minimize the pain they must experience. People are not compelled to avoid pain because of some Law of Pain Avoidance; this law exists because it is a good description of what people, acting of their own free will, usually choose to do.1


2 and 6. In both of these cases (plastic neurons and a friend’s neurons), there is no single point when “me” switches to “not me.” Individual identity is not that particulate. There is no way to say exactly when one individual ceases to exist and another comes into being. Even aside from neuron-replacement thought experiments, I am not sure how valid it is talk about anybody existing over time as exactly the same individual. The “me” writing this sentence now is closely related to the “me” that existed last week, but are we really exactly the same? Similarly, “me” and “me with three of my friend’s neurons” would not be exactly the same individual, but we would still be closely related individuals.

Rather than trying to pinpoint exactly how much change can take place before one individual becomes another, it is more accurate to think of individuals as instantaneous occurrences. One person, then, is really a continuous string of very closely related instantaneous individuals. Likewise, “me” and “me with one plastic neuron” are separate individuals more closely related than “me” and “me with two plastic neurons.”2

3. When comparing a natural neuron and an electrical transistor that behaves exactly like a neuron, it seems arbitrary to call only one of them a computer component. Is there really any difference between a human and a very, very complicated computer? This question makes me think of the first chapter of my biology textbook, which contains a section on the characteristics of life. My first reaction was to skim through this introductory chapter, but when I paused for a moment, I realized just how much thought must go into creating even a reasonably comprehensive definition of life. Even the best definitions we have been able to come up with—which point to characteristics such as evolutionary adaptation, energy processing, order, regulation, growth, and reproduction--have blurry edges. Viruses are an obvious example, but very sophisticated computers might also inhabit this gray region.3 Of course, computers are different from most life forms in some respects. For example, they do not contain cells or replicate themselves using DNA. However, they do have the potential to fulfill many of the characteristics of life. Theoretically, I also think that computers could someday be conscious in the same way that humans are conscious. Human consciousness does not have its origin in otherworldly fairy dust or any literal breath of the divine. Instead, it is an emergent property of the atoms, molecules, and cells that compose the human brain. These components of the brain operate according to the same natural laws as everything else. It is conceivable that advanced computers, built from the same basic materials as humans, could mimic human brains well enough to have consciousnesses similar to ours.

4. The basic question here is whether human identity is purely a product of the brain, or whether the rest of the body has some role. On the one hand, so much of personality, memory, and consciousness are based in the brain. People who lose limbs are still themselves; people in irreversible comas are not. This would imply that my brain in a robotic body would still be me. However, I do not think that this is completely true. While a large percentage of identity is certainly based in the brain, humans cannot be regarded purely as brains that just happen to reside in bodies. The relationship between brain, body, and identity is closer than that.4

5. Initially, both copies would be me, but as time passed, they would diverge into separate individuals with separate consciousnesses. This is similar to identical twins developing distinct personalities as a result of their different life experiences. (And while my current consciousness would be the common ancestor of the two copies’ consciousnesses, neither copy’s consciousness would be identical to what at this moment I think of as “my consciousness.”)5

7. The old, undivided-brain me would be gone; some new but related individual would exist instead. In this case, it is more accurate to say that the current me would be replaced by two new but related individuals, since neurons completely incapable of communicating with each other cannot fully be part of the same individual.

8. Personal identity is much less well-defined than we might like to believe, and certainly much less well-defined than we tend to treat it in everyday life. Identity is not a constant; it develops and changes continuously.


1. I’m not sure there’s equivalence here.  People can break the law of pain avoidance; planets cannot break the law of gravity.

2. This is essentially Robert Nozick’s closest-continuer theory which he also uses to address the ship of Theseus paradox.

3. This will also become a topic of considerable debate as humans approach the ability create “life” from scratch.

4. Antonio Damasio makes this point (the importance of bodily sensation in our conscious lives) in The Feeling of What Happens.

5. I agree and yet still find it difficult to not reflexively ask, “But which one will have my consciousness, the one I am experiencing right now?”