Argument from interaction
Clearly, for a soul to have a meaningful connection to the body, it must be capable of interacting with matter. Yet, souls are defined as immaterial and not subject to the laws that govern matter. Hence, the paradox arises: by its definition, a soul must be both capable of interacting with matter, and not capable of interacting with matter. To elaborate,
Matter affects matter through interactions. For example, you can push a desk, or bludgeon a man, or dig a river. It is because matter is so "interactive", that we can make measurements, conduct experiments, and observe phenomena associated with matter. The soul, on the other hand, is by definition immaterial. Hence, with our scientific instruments we cannot detect it. If we could detect it, we could then determine its properties and structure and we would be able to materially interact with it, which would make the soul material.
But that's a funny thing, considering that the soul is supposed to interact with the body. After all, we are only aware of our world through our senses; and our conscious decisions directly translate into physical actions -- e.g. if I wanted to clap my hands together, I could do it. So it seems that material information must have a way to enter the soul, and material information must have a way of emanating from the soul and travelling to the body.
The latter of these phenomena has a definite effect on the body, and hence must be indirectly detectable. This is because the body is indeed material, and any changes introduced within it are thus immediately detectable with proper instruments. Thus, were the soul to feed information back to the body, scientists ought to be able to find the spot where information from the soul enters the body for the first time. (Of course, despite centuries of searching no such spot has been found.) But this again contradicts the notion that the soul is not detectable through material means (of course, this contradiction arises out of the already contradictory notion that the soul interacts with the body.)
Then there is the question of the very mechanisms through which the exchange between the soul and the body takes place. By definition, a soul is 100% immaterial. On the other hand, the body is 100% material. How do we build a bridge between the two? Does there exist a "something" that is both partly material, and partly immaterial? But anything like that would not make sense, since the idealist concepts of matter vs. essence are incompatible. Matter is temporary, while the soul is eternal. Matter is corrupt, while the soul is perfect. Matter possesses extension, density, mass, color, temperature, etc. -- while the soul has none of those properties. Matter can be subdivided, yet the soul cannot. How can "something" exist that possesses a mix of these contradictory properties? How can something be corrupt and perfect at the same time? How can something be massive and massless, colorful and colorless, extended and shapeless? So it seems there is no reasonable way that the gap between the immaterial and the material can be crossed so as to enable the communication between the soul and the body.
To sum up, two distinct points are raised here: first, the definition of the soul and its relationship with the body are contradictory, and second, there is no satisfactory explanation of how the soul can exchange information with the body.
Argument from neuroscience
For the purposes of this argument, we must first determine that of all the body parts, it is the brain that makes us who we are. After all, you can take a normal human, amputate all of her limbs, and she will still be defined as a human being. You can take a human being and cut out his heart, lungs, kidneys, bowels, etc. and he would still be a human being (for as long as surgical machines can do the work of the missing organs.) If you cut off somebody's head, and somehow manage to keep it alive, then it's the head we would point toward when we discuss that person; the headless body will no longer be ol' Joe -- since here's ol' Joe's head that speaks in Joe's voice and thinks and feels like Joe, and possesses all of Joe's knowledge, etc. So we can keep imaginatively (and nonchalantly) stripping Joe of body parts until only the brain is left floating in a jar. At this point, we can still safely point to the brain and say that it's Joe; we can incinerate the other body parts, but as long as the brain is alive, Joe is alive too. Incidentally, that's why clinical death is defined as brain death. Any other failed organ can be replaced, at least in principle; however a brain cannot be replaced. Even if Joe clinically died, and you transplanted Brent's brain into Joe's skull, all you would have done is transplant Brent's persona into Joe's body; Joe would still be dead as a doornail.
Now then, it seems that the brain is the crucial part of us that makes us who we are. Incidentally, the brain also physically controls the body. If you want to bend a finger, a train of signals has to travel from your brain down your spinal cord and through your peripheral nervous system all the way to the muscles of that particular finger, so that they contract or expand so as to bend the finger in the way you wanted. If the pathway between the brain and any particular part of the body is breached even at one spot, you will lose your control over that part of your body. Hence, the brain is not only the defining part of what it is to be human -- it is also the part that actually controls the body! So, if the soul is to interact with the body, it is clear that the soul must interact with the brain.
But where in the brain does this interaction with the soul occur? It turns out that there is no possible answer. As you may or may not know, the brain can be crudely subdivided into an old brain and the new brain, the latter composed of the left and right cerebral hemispheres. The old brain consists basically of the brainstem, and in humans is more or less a mere interface between the new brain and the spinal cord, as far as cognitive function is concerned. This is not to say that the old brain is insignificant, since it contains physiologically crucial centers controlling everything from heart beats to breathing to sleep-wake cycles. However, it is the new brain that is responsible for any behavior that we would consider above comatose. The new brain possesses vast tracts processing and combining information from the five senses, it possesses structures that plan, initiate, and control movement, it possesses structures responsible for emotions, it possesses structures involved in memory, attention, spatial navigation, object recognition, production, perception, and comprehension of speech, etc, etc, etc. In fact, brain damage studies show that every last bit of the new brain in adult humans is involved in at least one, and often several, cognitive tasks. So, it would seem that the soul must be in contact with the entire brain if it was to account for all of our human faculties. However, this does not hold when we consider abnormal physiology.
Certain birth defects cause some children to be born with only one cerebral hemisphere; other children lose a hemisphere to surgical intervention very early in life. Despite the fact that for an adult to lose a hemisphere would be absolutely devastating in terms of loss of function and aspects of personality, these children grow up to be nearly normal in all respects. This is just one example where the amazing plasticity of the brain shows itself in full glory. Thing is, the plasticity is lost early in life as the brain becomes increasingly organized, since for a highly structured brain plastic change would actually mean loss of function rather than gain. Yet, the very fact that people are alive who function normally with only one hemisphere (and a brain that is organized vastly differently!), as opposed to the "normal" people who have two hemispheres and a totally different brain organization -- poses difficulties for any proposed mechanism of interaction between the soul and the brain. Already, it would seem that the mechanism is not dependent on the soul, but must adapt to the developing brain on-the-go, so as to connect the soul to the brain correctly, whatever the final architecture of the adult brain may be.
The functional portion of the brain is composed of vast and very complex networks of a total adult average of 10,000,000,000 special cells called neurons (the bodies of these cells contain pigment and are often collectively referred to as "gray matter"). Each neuron sends out slender connections to other neurons, and an average neuron is connected to about 10,000 others (these interconnection fibers are wrapped in other special cells that form an electrical insulation around these "wires"; as a result the connections look white to the eye, and en masse are referred to as "white matter"). Of course, there are trillions of other cells in the brain besides neurons, which compose blood vessels, provide insulation and scaffolding for the connections between neurons, nourish neurons and clean up their waste, fight invading pathogens, etc -- but neurons are what actually does all the work of cognition. Neurons work by sending electrical impulses to other neurons, and accepting similar messages. Without going into too much gory detail, the effect of the messages on any particular neuron is mediated by a slew of factors from the actual chemicals used to pass the message between neurons, to the actual characteristics of the voltage signals that neurons send to each other. But the great and overriding point here is that neurons are literally billions of independent cells, communicating among each other, and every now and then sending impulses through your peripheral nervous system to affect what your body does. It seems that to control the body, the soul would have to connect individually to every last neuron in the brain and control what it does. But neurons die all the time, and new neurons are born also (although at a much slower rate.) Furthermore, the actual connections between neurons change constantly, and so the role any particular neuron plays in the overall function of the brain varies with time. So, how does the soul know what each neuron's current function is? Additionally, it seems that scientists can predict neuronal behavior precisely, based purely on the electrochemical impulses it is receiving from other neurons. So it appears that there is no mysterious soul behind the curtains telling this neuron to fire and that one to hold off once every millisecond; behavior of neurons is determined exactly by the input they receive from other neurons. And some of those other neurons receive a lot of their input from sensory organs, such as the pressure, pain, temperature, etc. (in other words, somatosensory) receptors on your skin and other organs, or from your eyes, ears, nose, or tongue, or from the vestibular apparatus in your inner ear, etc. So it seems that the brain is a deterministic machine that is driven by inputs from its environment. And all of those receptors and organs have also been studied in detail, and found to be purely biochemical and physically deterministic. There is no place left for the soul to operate!
There is no end to the problems that neuropathology brings for the soul, and I am not going to attempt to list even a small portion of such problems. However, I already mentioned the conundrum posed by neural plasticity. I'll present just one more "problem", and then move on to the next argument. The problem has to do with the split-brain patients.
Some people are subject to debilitating seizures, which are uncontrollable through drugs. A seizure is really a runaway chain reaction where a bunch of neurons starts firing chaotically, and the chaos spreads across the cortex, disrupting any cognitive function in its wake. Seizures can sometimes be combated through drugs, which help regulate neuronal activity and stop it from crossing a vital threshold above which it spins out of control. Newer methods include electrodes implanted directly into the particular brain region where seizures originate, so that an implanted computer can detect an onset of the seizure and apply a mild electric current between electrodes, which in effect "resets" the surrounding neural tissue and stops a seizure in its tracks. However, a while ago such advanced treatments were not available, and in extremely debilitating cases the only recourse was surgery. Most often, the small brain region where seizures originate was surgically removed (the mild loss in cognitive function was a small price to pay for the freedom from frequent seizures, and was especially tolerable for children whose brains are still plastic enough to compensate for the injury). However, in a few cases the offending region was crucial to certain treasured faculties, such as for example production or comprehension of speech, or control of posture. In other cases the offending region was just too large. In these cases, the surgeons did the next best thing to excising the part of the brain -- they selectively cut some of the connections between this brain part and other parts of the brain, so that the seizures would only occur locally and would not spread.
Seizures can occur in relatively localized regions of the cortex, but for some unfortunate people they occur globally, spreading from one hemisphere to the other like wildfire. In these cases, where excision was not an option, surgeons used to sever the huge bundle of fibers (called "corpus callosum") that connects the right hemisphere with the left. The corpus callosum is the major connection between the hemispheres, and although there are other small communication channels via which certain parts of the two hemispheres exchange information, when the corpus callosum is severed for all practical purposes the hemispheres are cut off from each other. For this reason, the patients that underwent this type of surgery came to be known as split-brain patients. And they permanently exhibit the weirdest behaviors. They really do have two separate, almost independent brains in their skull. Most of the time, the brains coexist peacefully. However, sometimes they don't agree with each other and the results can range from comic to absurd to horrible.
Because of the way the brain is wired up to the body, each hemisphere controls the opposite half of the body. So, the right hemisphere controls the left arm, leg, etc., while the left hemisphere controls the right half. One patient had a problem with his left hemisphere: apparently, it just couldn't stand his wife. At the mere sight of his spouse, his right hand would immediately form a fist, his right leg start making valiant attempts to get the body closer to the wife, and his right arm start violently swinging at the wife with a clear intent to do damage. With his left leg he would fight his right leg, and with his left hand try to restrain his right hand, all the while displaying a grimace of rage on the right side of his face while the left side of the face expressed clear alarm and distress. Another lady had an even more serious problem, with the two halves of her body engaging in a vicious feud. She literally beat herself up, tried to choke herself in her sleep, tore her own hair out, and all of that occurred in the context of the right side of her body doing damage to the left side, and vice versa. Fortunately, such horrible side effects tend to mellow out as time passes, but the patients never return to normal -- to the end of their lives, they literally remain split in half. Yet, if a single, indivisible, unified soul was controlling the brain, then surely cutting the link between the hemispheres would not preclude them from functioning in harmony! At the very least, they shouldn't be trying to kill each other! But contrary to all common sense as we used to know it, the two hemispheres literally turn into two distinct personalities. Each of them is capable of independent emotion, independent knowledge, and independent interaction with the world. For example, questions can be asked of the right hemisphere, and it will answer them (though not verbally, because in most people the right hemisphere is incapable of language) -- but the left, verbal, hemisphere will never know about either the questions or the answers, and will in fact tell you so when asked. Even more poignantly, the right hemisphere possesses knowledge that the left hemisphere doesn't, and vice versa. Both hemispheres exhibit structured thought and problem solving abilities, independent of each other. Both of them express feelings and emotions, again independently of each other. Each has its own stream of consciousness, again independent of the other hemisphere. So indeed, the two hemispheres are in most respects separate, distinct, independent human beings! Yet, they originally only had one soul. How would the doctrine of souls explain such a phenomenon?
Yet another difficulty lies in the transfer of memory or knowledge between the brain and the soul. For example, you might remember what you did during the last Christmas, and when asked you would tell us a story describing what happened. This process of recalling facts and then verbalizing them involves many crucial faculties that are just about as central to our stream of consciousness as anything -- so presumably at least a large part of the process occurs in the soul and not in the physical brain. However, it is well known that the brain contains certain regions specifically dedicated to memory. When these regions are damaged, the result is amnesia -- loss of memory -- despite the fact that all other cognitive functions remain intact. Now, what happens when an amnesic is asked to describe something they knew prior to the brain damage, but of which they now have no recollection? The request gets correctly processed and understood by the subject, as can be verified by questioning him about it. Presumably, such higher understanding resides in the soul, so the soul indeed knows what is being asked. The patient is also perfectly able to verbalize other facts, and to tell stories not connected to the particular lost memory -- so these faculties are preserved as well. Therefore, if the soul still retains the memory whose representation is lost in the physical brain, it should have no problem verbalizing that memory and telling stories about it, and thus in fact amnesia would never even be observed! Yet, amnesia is real and very predictable based on which regions of the brain are damaged. So, it seems that destroying a part of the physical brain utterly destroys the memories it used to help encode. This means that the soul does not possess memory; memory is purely a property of the brain. Which means that when the brain dies, all memories die with it. Which means that the entire personality dies with the brain, since memory includes, in addition to explicit facts, everything from learned skills such as language, coordinated movement, or art, to such things as preferences, attitudes, beliefs, etc. Which comes into a huge clash with all the claims of afterlife where the souls are supposed to retain memory of earthly existence and even maintain their pre-death personality.
Argument from neuropsychology
This gets to the reason why we conjecture the existence of the soul in the first place. In the old times, when people knew very little about the nature of life or cognition, it baffled them that certain objects were indeed alive, and other weren't. It baffled people even more why certain living creatures, such as humans, have civilizations, art, language, religion, etc. while other living creatures have none of the above. People also wondered what happened to them when they slept, as they often seemed to depart the regular world for other bizarre realities, inhabit bodies other than their own regular body, fly, and do all sorts of amazing things that other normal things just aren't seen to be doing. And then, people wondered what it would feel like to die, and what happens to their friends and family once their bodies are destroyed, and they also wondered where their stream of consciousness came from, and how come they can't remember anything prior to their early childhood. Thus came around suggestions that what all life has is something special, some kind of a "living essence" that separates it from non-life. You will find that particular idea in every single culture that ever existed, which goes to show just how natural such a conjecture is, and how easily it arises. It may have been a reasonable suggestion, until relatively recently when science began to unravel the true mechanisms of life and cognition.
Today, we know that the simplest forms of life contain no "living essence" at all -- they are merely very complicated chemical structures that are able to obtain energy and material from their environment, and to reproduce themselves. Thus, in one deft blow the pre-existing void between matter and essence is bridged. It stands to reason that if unicellular life does not possess a soul, same holds for multi-cellular life -- since multi-cellular organisms are nothing more than intricately organized and coordinated colonies of single specialized cells.
But what of the stream of consciousness, the emotions, the awareness, the sensations, the knowledge, the reasoning power that we all possess as humans? How do all of these weird qualities derive from mere cells? Well, the answer has not yet been entirely completed, and I personally hope to play a part in completing it. But the beta version goes something like this.
In what may at first glance appear to be a grotesquely oversimplified analogy, consider modern computers. What you see on your screen is a pretty complex visual image representing an attempt at a simple, elegant, and easy to comprehend User Interface. Behind that interface lies complex functionality that enables you to create documents, exchange information with other people, play games, create art, listen to music, render computer movies, simulate collisions of galaxies, analyze data, design other computers, and in general do an amazing variety of things. Most of those applications depend on arcane algorithms and complex protocols to work, of which you as a user have no knowledge or comprehension; all you work with is a friendly (or at least not as arcane as the source code) UI, which abstracts you away from all the hair-raising complexity that dwells on your CD-ROMs and inside your particular beige box.
The brain presents a somewhat analogous picture. What we observe is the outside, equipped with a "user interface" consisting of the body. We can interact with the body, we can communicate through it to the brain, and receive replies from the brain through the body. In essence, the body abstracts the brain from us, and as generic "users", we are not aware of how exactly the brain does what it does -- nor do we particularly care, as long as the brain does its job, and does it well. However, the analogy with computers is not complete, since whereas with computers we at least have engineers and programmers who understand exactly how the computer does the things it does, with the brain, at least at the outset, we possess no such knowledge. Thus, the problem of figuring out how the brain works can be compared to the following hypothetical situation: imagine that the enlightened ancient Greeks happen to chance on a complete modern computer system, loaded with all the software, connected to an uninterruptible power supply that will last for decades, and programmed so that its user interface is in ancient Greek (so they can at least partially decipher what it is that it does.) Now imagine just how hard it would have been for the poor Greeks to figure out how all that graphical splendor and functionality arises from that box cluttered with weird metallic and non-metallic parts. Heck, they'd have to develop the theory of quantum mechanics before they could understand how a single transistor works, and they'd have to develop ultra-powerful microscopes to even find those transistors. They'd have to develop an entire theory of computation before they could understand how the mysterious box is able to exhibit such strangely life-like interactivity. Then, they would have to reverse-engineer all the circuits of the computer, and understand exactly how they interact and tie together into a working system. Then, they'd have to reverse-engineer all the binary machine code on the computer's hard drive, and determine how it affects the CPU and other components to do the things that they do when various programs are run. Then they'd have to find ways to de-compile the machine code into a human-readable language, so that they may finally understand how the programs are put together, and how they work. Only then will they finally understand that the computer is not a magic or cursed item, that it is not a living organism or a gateway into another dimension, that it is not a God in disguise and not a fundamental key to all creation -- but that it is what it is, a machine that processes information according to certain pre-set algorithms.
An equivalent claim is made for the brain: it is a machine that processes information according to certain pre-set algorithms. And we face a horrendous task of reverse-engineering the brain in order to understand it, in a way very similar to the plight of the unfortunate ancient Greeks. Only the brain is even more daunting than the most complicated computer in existence. It sports an equivalent of 10,000,000,000 processors interconnected in complicated ways, all working simultaneously at 50 Hz in a cacophony of communication. It is fluid, and constantly changes its very structure. It computes not only with electricity, but also in a large way with biochemistry, which makes the behavior of its individual CPUs much more complicated to unravel than the behavior of a typical circuit. It is inexorably tied to the body throughout its development and function, and so to understand the brain we must also understand the workings of the body in all of their intricate detail. The brain is shaped by genetics as well as sensory and chemical input as it develops and matures, so we must understand all of those processes with a high degree of confidence and in great detail over time spans lasting well over a decade from birth to maturity -- if we are to understand how the brain acquires its structure and generates its circuits. And then, once we unravel the story of the hardware, we must understand how it translates into the actual behaviors that we observe -- in essence, we then must reverse-engineer the brain's algorithms and put them into plain English before we ever hope to claim that we completely understand how the brain works. The task is clearly not for the weak of heart. In fact, it can be argued that unraveling the human brain is among the few most difficult challenges science has ever faced. And the task will clearly take at least decades, if not centuries, to complete. But we are already making the first brave steps, and so far we have learned enough to very crudely describe what lies behind our various and wonderful cognitive powers.
In the course of our studies, we have localized regions of the brain, or "nuclei", that either by themselves or in concert with other nuclei directly correspond to various human faculties. For example, there is a clearly defined subsystem in the brain that is linked to emotion. Lesioning the lymbic system will turn a person into an automaton incapable of generating or expressing absolutely any kind of affection for anything. Such patients even talk in rhythmically perfect monotone, like robots from cheap sci-fi flicks. As another example, the memory subsystem has been located in another brain structure, the hippocampus and the parahippocampal and entorhinal cortex regions. Damage to these areas predictably results in various forms of amnesia, with the exact symptomology dependent upon exactly which parts of the system were damaged, and how extensively. As another example, take the ability to understand spoken speech. This capacity is at least in part dependent on a part of the cortex called Wernicke's area, damage to which instantly turns the speech a patient hears into meaningless gibberish, and has the same effect on the speech actually produced by the person (though they are not aware that they make no sense to the others; in fact they are usually quite distressed at the fact that the others are talking gibberish and can't understand what the patient is saying.) Amazingly enough, in a fully organized adult brain there even are regions devoted specifically to reading written text, or specifically to writing text. Damage to these regions results in strange symptomology, such as for example a person being able to read, but no longer able to write, or being able to write, but not being able to read back what they just wrote. Such study of neural pathology has produced an innumerable flood of findings like these, and the deluge has yet to show signs of subsiding.
Additionally, computational modeling and animal research have been providing insights into other crucial powers of cognition. For example, the faculty of vision has been, is, and will be studied with utmost intensity. As examples, we have discovered cells in the brain that respond to lines of various orientations in the visual field, or variously oriented and scaled gratings of alternating light and dark regions; we have found cells in the visual cortex that respond to local motion in a certain direction, or to a contraction or expansion of the local texture (indicating approaching or receding objects); we have found cells higher up in the processing hierarchy that combine those basic features into more complex items, such as corner, or circle, or crosshatch patterns, and we've found cells yet higher up that respond to entire objects only of a certain type, such as faces for example. We've tentatively began to trace the diverging pathways in the visual processing stream, where one pathway specializes in recognizing objects, while the other pathway specializes in determining the location of objects in space around the observer, or the observer's relative coordinates with respect to objects. We are currently constructing rather successful computational models of how rats tell where they are, based exclusively on the rat neurophysiology and actual electrical recordings from individual cells in rat brains. We have constructed a very successful neuro-computational explanation of how barn owls determine the direction of the sounds they hear. People are digging in on all levels, from planning, coordination, and initiation of motion, to hearing, somatosensory perception, mastication, memory, emotion, mechanisms of attention, to cognitive and neurobiological development, to language, etc, etc, etc. Slowly but surely, the brain's enigma is giving way and grudgingly surrendering territory. And absolutely at no point anywhere within this extensive and burgeoning research field has any research group ever found even a remotest hint of anything supernatural.
But what we actually do, at this time, know about the link between brain and cognition -- is that the various cognitive faculties that in the past could not even be imagined to stem from mere matter, derive from specific regions in the brain, and the relationships between these brain regions and how linked regions combine to create cognition, are very physical and well-defined indeed. Additionally, severe damage to a brain region (in adults) connected to some cognitive ability completely and permanently destroys that ability; no hint of its past existence can be recovered through the use of other faculties, as should have been the case if the "lost" faculty actually resided in the soul.
Argument from evolution
Well, this one's short and sweet, and will work against only a narrowed selection of various doctrines. It basically says that since, obviously, simple life forms do not have souls, and we are merely evolved forms of the same thing, then surely we don't have souls either. At a deeper level, the argument challenges the believer to define at which point living beings acquire souls. Do only humans have souls? But then you have problems with primates, since they are so incredibly similar to us both physically and behaviorally. Do only primates have souls? But then you have a problem with the simians, since monkeys are so similar to apes both physically and behaviorally. Do only primates and simians have souls? But then you have a problem with the prosimians, etc, etc, etc. Eventually, you are forced to retreat to a generalization over all mammals, then over all animals, and finally over all life -- at which point you arrive at a stark contradiction with a clearly observable fact -- that the lowest forms of life do not have souls.
Argument from development
This is somewhat similar to the argument from evolution. Here, you are challenged to define just at what point during development a human acquires a soul. It couldn't be at the point of egg fertilization, since at that time everything is still purely biochemical, and the fertilized embryo has no properties normally associated with a soul. It couldn't be during early embryonic development, since an early human embryo is anatomically and functionally indistinguishable even from fish embryos. So when is it that a human acquires a soul? The answer to that question is impossible similarly to how it is impossible to define a cutoff across different lifeforms -- because just as the spectrum of lifeforms on earth is fairly continuous in terms of their capabilities, form and function, the development of an embryo is similarly continuous. At no point during development does the embryo suddenly make a quantum leap and exhibits some feature it didn't have a second ago. This continuity makes it impossible to define a cutoff at which the soul definitely must be there. From another (and more mathematical) perspective, since a fertilized egg has no soul, then by induction over this smooth continuum of development we arrive at the conclusion that even a fully developed adult human doesn't have a soul.
Argument from objectivity
This calls into question the very need to have a concept of souls or afterlife. Neither are objective, in that neither are tangible, measurable, or independent of observer (e.g. neither can be detected by "brainless", mechanical scientific instruments that don't have a propensity for misinterpreting things like humans do). Neither can be tested, neither provides any tangible evidence for its existence. In fact, if one starts out with a (still futuristic) complete physical explanation of cognition, then one is not going to be likely to conjecture the existence of souls or afterlife -- simply because there would be no remaining evidence available that would prompt such a conjecture. Hence, objectively, the theories of "vital essence", or souls, or afterlife are outdated and superceded by modern science. As any invalidated theory should, therefore, the ideas of soul or afterlife properly belong in the history books, but no longer in the domain of serious discourse.
Argument from equivalence
This is where we assume that the brain has, at some point in the future, been scientifically unraveled to the point that absolutely everything is known, understood, and explained about its form and function. Then, we can imagine that the scientists of the future endeavor to replicate a complete human brain, but not in flesh in blood, but as a program running in some blindingly powerful supercomputer. The brain is simulated down to the last atom, complete with information input from simulated eyes that mimic human eyes, and simulated ears that mimic human ears, and all other sensory modalities equally well implemented, with a simulated body providing feedback to the brain, and a simulated ultra-detailed environment for that body to roam and interact with. Because the simulation replicates the function of a real human brain to the last detail, and it replicates a realistic environment for that simulated brain to mature in, the simulated human will certainly develop its own conscious stream of awareness, learn the details and workings of its environment, exhibit emotions, intelligence, sensations -- it will be altogether equivalent in all of its functions to an actual physical human. But it is painfully obvious that the simulated human does not have a soul, because in reality he is nothing but a pattern of bits in the memory banks of our supercomputer. Ironically, if we were to simulate not one such human, but an entire tribe living in some virtual jungle, and allow the simulation to progress across many generations, the humans will develop language, culture, and even religion, and likely one of their first metaphysical conjectures will have to do with the fundamental distinction between life and nonlife -- the "vital essence".