Atheroy,
Identical twins is just one problem for those who believe in the gibberish of souls and even M*Ws concept.
Here is a small portion of a well-written post from a past member (Boris) which highlights some of the vast problems for those who would like a soul to exist. Enjoy.
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.