If you take a calculator, and type "2 + 2 =", it will, assuming it's functioning correctly, report "4". But does the calculator know that 2+2=4? Does it have a subjective awareness of that fact? Few would argue that it does. But there clearly is something about humans (and quite probably a lot of animals) that can know things, like the fact that 2+2=4.
Back when I was at Catholic high school, in Theology class the teacher used this example to illustrate what he conceived of as a soul. The soul was the "thing that knows", that is aware. It was impossible, he advanced, that a material object, that "just stuff", could give rise to such subjective awareness. Brains might differ in their capacities to generate ideas (some brains might have trouble with math, and others do well), but the awareness of those ideas rested in the soul.
This fits in with childrens' naive ideas about the relationships between brains and minds:
"I can illustrate this with a story about my son, Max, when he was six years old. We were having an argument because he had to go to bed. I told him, You have to go to bed, it's very late, and he said, You can make me go to bed, but you can't make me go to sleep. It's my brain! This got me interested. I said, Okay, fine, stay up, let's talk. I got out a piece of paper, and started asking him questions about what he thinks the brain does. I gave him lists of things - does the brain do this, does the brain do that?
His answers showed an interesting split. He agreed the brain is involved in perception - in seeing and hearing, in tasting and smelling. He'd been taught that. And he agreed that the brain is particularly important with regard to conscious problem-solving: Solving a math problem, making sense of a story, planning what to do. But he said the brain didn't do certain things: it didn't do dreaming, it didn't do loving his brother, it didn't do pretending to be a kangaroo. Max said, that's what I do, though my brain might help me out."
However, this doesn't seem to hold up in practice. We don't know exactly how a physical object like a brain gives rise to subjective awareness - but we can be quite confident that it does. The key line of evidence is the simple fact that damage to the brain doesn't just damage "mechanical" capacities like math ability, coordination, speech, and the like - it also damages awareness, and does it in fundamental ways.
Consider aphasia. One type, Broca's aphasia, seems to fit the naive model. Sufferers are generally able to understand the speech of others, but have great difficulty speaking themselves. But then there's Wernicke's Aphasia. Victims can speak fluently, but comprehension is gone. They speak in what has been termed "word salad"; a stream of meaningless gibberish. If you put two patients next to each other, they may have an entire conversation of nonsense. And they won't realize it. Sufferers of Wernicke's aphasia not only don't understand language, they don't understand that they don't understand. Despite being unable to communicate verbally, they usually seem entirely untroubled by it, or, indeed, to even notice something's missing.
Then there's hemisphere neglect. A sufferer loses, say, the concept of 'left'. They ignore things on their left; they may eat only food on the right side of their plate, be unable to open a door with the doorknob on the left, etc. Ask them to imagine walking down a street, they will describe only the items on right side. Ask them to imagine turning around, and they will suddenly forget the items they were talking about, and start describing the other side of the street. Oliver Sacks (see below) notes a case of a man who had a stroke that massively damaged his visual cortex. He not only lost the ability to see, he lost the whole concept of sight; he couldn't even imagine seeing, or remember what vision was. He couldn't understand why everyone was concerned. Anyone who's seen the effects of Alzheimer's Syndrome has plenty of evidence that the brain is what's critical to the self. In the late stages it's hard for anyone to say that what's left is the original person in any meaningful sense.
After reading up on neurology, I can't really come up with a type of awareness that isn't eliminated or profoundly impaired by some kind of brain damage or another. Frankly, I can't see that there's much of a job left for a 'soul' to do. And I'm certainly not terribly concerned by what happens to my hypothetical 'soul' after I die... how can that, without my brain, be called "me" in any significant way? (It's the same reason why I'm not too comforted by the idea of reincarnation. If "I" come back, but without my memories, how can I really be said to have "come back"?)
Now, of course, there's the question of how the brain, a lump of matter, gives rise to self-awareness. My answer is simple: "I dunno." But even if a caveman didn't understand how a car's engine works, he could determine that it was the source of locomotion. We don't yet know how the brain gives rise to consciousness, but we have plenty of evidence like the above that it does.
I suspect that part of the problem is that we're looking at the question wrong. I think consciousness isn't a static 'object'; it's an active process. Consider wind - what is wind, exactly? It's something air does. Is a tornado an object, or is it a process that happens within a particular volume of air? I think the mind isn't something the brain has or creates - the mind is what the brain does. Or, as Frank Zindler put it, "To believe that consciousness can survive the wreck of the brain is like believing that 70 mph can survive the wreck of the car."
Self-awareness in humans comes about from the actions of billions of neurons in the brain. Clearly it's not just a matter of how many neurons are involved - imagine a trillion neurons, arranged end-to-end. They might be useful as a signalling line, but even though there's a trillion neurons, it's clear they have no significant information-processing capability, and are therefore nonsentient. Arrangement matters. We don't know (yet) exactly what neural arrangments are necessary for self-awareness, but clearly the human brain is one such arrangement. There may be other possible arrangements, that use fewer neurons, or that use entirely different fundamental units (transistors?).
Based on the above considerations, I have to conclude that something without a brain isn't aware or conscious. If someone's brain dies... well, even if their body can be kept operating I have to conclude that they are dead. And if brains are needed to be a living human, then it follows that before the brain forms, a fetus is not yet human.
If you think a fertilized egg has a soul, you run into problems with actual biological reality. Zygotes can split up to two weeks after fertilization, leading to identical twins or triplets, or even quadruplets. Let's keep things "simple" and consider just identical twins. Which twin has the soul? Or does each have half a soul? Or does God 'supplement the soul supply' in such cases with an extra? If so, when? At the moment of splitting? Or does it happen earlier - does God, at conception, infuse an extra soul in a zygote that It 'knows' will split later? Is there an objective way to detect zygotes that have an extra soul waiting for the split? Or if the extra soul comes later, is there an objective way to tell which twin had the original and which one got the 'bonus soul'?
Then there's the case of 'chimeras'. Sometimes two eggs get fertilized at the same time. If both manage to implant, you get fraternal twins. But (very rarely) sometimes those developing zygotes fuse, and give rise to a person that is composed of two different cell lines. This has resulted in puzzling cases where a child appears not to be genetically related to the mother that conceived and gave birth to them! Consider, if both zygotes got souls at the moment of conception, does a chimera have two souls? Or did one of them 'die' and the other one kept on living? (Which one? Did the one that died get a 'free pass' to heaven?)
Needless to say, I find this kind of argument to be a lot like the whole "how many angels can dance on the head of a pin" argument. Some people like to point to conception as the 'objective' start of human life, and in some senses that's even correct. But you just can't use words like 'soul' and 'objective' in the same sentence. If you think so, provide some testable answers to the above questions. All I've ever heard from those who believe in souls boils down to, "Gee, I dunno."
Given that brains are needed for self-awareness, at least in humans, it's awfully hard to see how a clump of undifferentiated cells clinging to the wall of the uterus could be a self-aware person. While this may be a potential person, it's not an actual person; and potential people have potential rights, while actual people have actual rights. In the very early stages of development, I don't have any major problems with abortion, though I think it's regrettable.
Now, the basic brain forms pretty early on, like within a couple of weeks. Clearly at some point self-awareness develops. But when? What's the minimum size and complexity of a brain needed to support self-awareness? I'm not sure there is a sharp dividing line. Consider - at sunset, when, exactly, does 'day' become 'night'? What is the last nanosecond of 'day'? What's the first nanosecond of 'night'? And why? Does one follow the other immediately? Or is there a period of time that isn't day or night? Similarly, I'm not sure that there could be a clear dividing line between 'personhood' and 'not-personhood'.
'Slippery slope' arguments are usually bad ones. It's rare that someone can point to clear, unquestionable cases of 'slippery slopes' where allowing even a little leeway results in major mistakes. However... the question of "who's human" is definitely one of them. The Holocaust, innumerable war crimes, slavery, the subjugation of women... failing to extend full moral significance to others really does lead to tragedy. So I understand when people passionately object to abortion. If you believe that even a fertilized zygote is a human being with full moral significance, then abortion really is a massive tragedy.
And I don't think I'm somehow automatically immune from dehumanizing others. It's a sadly human trait. I have thought very seriously about this. It's important to be very cautious about the definition of 'human'; history shows all too clearly that if there's doubt, better to be too broad than too narrow.
But just because there are difficult boundary cases doesn't mean that there can be no certainties at all. Wherever you come down on the subject of twilight, high noon is definitely day and midnight is definitely night. If the Sun is on the other side of the Earth from you, then it's night. And if there's no brain, I don't see how there could be a human person there. In the case of Anencephaly, I don't think there ever was a person. And a fertilized egg, or even a blastula, does not have a brain. Unless they suffer from anencephaly, eventually a fetus does have a brain.
It's important to note, though, that just having brain tissue isn't the same thing as having a brain. The cerebral cortex starts forming within one month, but all the connections to the rest of the brain don't fully form until almost 30 weeks. Indeed, it seems that 20 weeks is about the minimum point that a fetus could possibly be aware of something as basic as pain.
My best guess is that anything like we'd call 'awareness' - let alone self-awareness - depends on a cerebral cortex of substantial size, interconnected with the other parts of the brain. Still, it's best to be conservative, as I noted above. I have a hard time coming up with a coherent case for banning abortion before 20 weeks or so - based on what we know of how the brain works and develops, summarized in the link above, there doesn't seem to be a way the fetus could be aware of anything, could be a person or agent in any meaningful sense. After that point, though, there's at least a solid case to be made that aborting the fetus harms an actual person.
It's worth noting that a tiny percentage of abortions actually happen after five months. By that point the mother has usually decided to carry the baby to term. In almost all cases, abortions after 20 weeks are for medical reasons.
I'm more troubled in the case of rape; but again, in practice a rape victim knows she's pregnant early on, before the brain has a chance to form - definitely before 20 weeks. Incest is a different problem; while there's a higher likelihood of genetic problems, that's no automatic reason to deny the child a chance at life. (Incestuous rape would come under the heading of regular rape.)
What about threat to the life of the mother (again, the vast majority of abortions after 20 weeks are for just such medical reasons)? At that point I think it's her decision. There is no legal obligation to force someone to risk their lives to help another. (A champion Olympic swimmer can watch someone drown in a river and never lift a finger; he can't be charged with any wrongdoing, at least in the U.S. People may (and, personally, I think should) think less of him, but he didn't break any laws.) Similarly, if there's a substantial risk to the life of the mother, I think it should be her choice whether to proceed with the pregnancy or not.
Studying embryology shows that while we can draw some well-supported lines, we don't know enough to draw a line between when a fetus is 'not aware' and when it is 'aware'. Similarly, there are very tough cases where we can't tell if awareness has ended. In general, I think that things like living wills should be respected. In cases where there's no clear will, the wishes of the legal guardian should be respected, though I'd prefer erring on the conservative side.
For example, in the highly-publicized case of Terry Schiavo, I'd say that the husband should have simply given custody over to her parents. If he was right and she's truly brain-dead, then it couldn't harm Terry and would make her family feel better. If he'd been wrong, there'd be a chance therapy might help. If no one was willing to assume custody, I'd say that it was up to the husband what to do.
Some people seem to assume that if there is no soul, if consciousness ultimately arises from a physical process, then it must be inherently valueless and meaningless. How can we be anything special?
In answer, I ask them to consider a rainbow. A rainbow is not the product of fairies painting the sky or whatever. A rainbow is the result of billions of raindrops of just the right size being illuminated from just the right angle. The light reflects inside the drops a couple of times and, in the process, is split like in a prism. That doesn't make a rainbow any less beautiful, though. A rainbow is not 'degraded' by having arisen from 'mere' physical processes. Physical processes are ennobled by giving rise to such beauty.
We all recognize that arrangement matters critically. A bunch of flour, sugar, butter, eggs, chocolate powder, and so forth can be made into a delicious chocolate cake... or an inedible mess, depending on who's doing the baking. And a baker can know precisely what is in a cake that they made, and exactly how it was made... and still enjoy eating that cake. It is true that the Mona Lisa is 'just an arrangement of pigments on some canvas' - but that arrangement is deservedly famous.
People are amazing, wonderful things. They are not 'just bags of chemicals'; each person is an extraordinarily special, literally unique pattern and process of chemicals. The raw materials aren't special, it's the arrangement thereof that's unique, valuable, and irreplaceable. We can know what people are made of (apparently entirely) and still find them precious and wonderful.
I don't have less respect for people because they are, at root, 'physical processes'; I just have more respect for what physical processes are capable of. It's true that humans are, in some sense, 'animals' in that they are related to all other life on this planet by common descent (which is a profound thought all by itself). But we are also unique among the species on this planet, in our ability for language and complex thought. We are different in kind and not merely degree from all other 'animals'. To me, at least, that's as special as any 'soul'.
There's another important difference between humans and other things, even complex things like snowflakes and rainbows - humans are aware, and self-aware. It's the difference between a subject and an object. You can damage an object (like a rake, say); but you cannot harm an object. You can only harm a subject, something with awareness. That makes a major moral difference. (Yes, atheists atheists can have morality too.)
Perhaps I don't hold anything 'sacred', in the sense of "designated or exalted by a divine sanction". But that does not mean that there's nothing I hold to be worthy of respect, or admiration, or even awe and veneration.
(Of course, the whole question of the "meaning of life" has other problems, as detailed here.)
We don't know that right now. This is one of those questions (like the origin of the universe) that I don't think we've had the right insights to understand, yet. We're like the people in the Middle Ages who saw a meteorite hit the ground. Clearly this rock came from the sky, but from where? Why hadn't it fallen before now? What held it up before that?
I think we will eventually figure this one out. For now, I just have to accept that it's something we don't fully understand yet. We do have some inklings of how it might work; see Further Reading below.
Sort of. For one thing, as a computer type I may have a higher opinion of what 'machines' are capable of than others. But the other problem is that, while in principle that might (note: might, see below) be true, in practice that's not really the case.
First off, we already accept (via Quantum Mechanics) events that are, in a deep sense, nondeterministic and random. According to QM (which has been confirmed, in detail, over and over again by experiment after experiment) quantum events are inherently random and not controlled by secret 'hidden variables' and such. Some people theorize that consciousness is deeply rooted in QM, and that would make people's actions not fully predictable even in principle.
But even if QM has no direct effects on consciousness, there are other reasons why people wouldn't be predictable automatons. One of the most important is chaos theory, where certain types of systems (and portions of the brain have been shown to be of these types) are 'deterministic but unpredictable'. Read up on the link if you want the details. One key point is that the behavior of the system is sensitive to tiny changes in inputs and states. To predict the weather accurately beyond a week or so, you would literally need to know the exact state of every atom on Earth (and, indeed, the Moon, and probably most of the solar system).
The key point is that, in practice, there's no way to tell the difference between someone with 'mystical free will' and someone whose brain is fundamentally based on chaotic and/or QM principles. They'd behave, in all practical ways, indistinguishably.
So, my attitude is that I don't fret about whether I have free will. No one would ever be able to prove that I don't have free will, anyway. Moreover, if I do, I don't have to worry about it, and if I don't, there's no point in worrying about it.
These are challenging and fascinating books, which will expand your conceptions of what brains and consciousness are, and are capable of. I guarantee it. You don't have to agree with everything in them (I don't) but if they don't make you think, there's probably nothing that can.