The overall point of this discussion is to show that Chazal's views and rulings on life and death were fundamentally related both to the mistaken beliefs of the era about physiology as well as the limited medical possibilities. Thus, any halachic analysis of this topic must take this into account in order to be valid. Furthermore, our own understanding of physiology, together with the medical possibilities available to us, mean that brain death should be defined as halachic death.
1. Chazal believed that the heart and kidneys are the seat of the mind and free will.
At least some of Chazal - probably most or all - believed that the heart and kidneys are used as the mind and for making decisions (free will). Prooftexts are as follows:
The Rabbis taught: The kidneys advise, the heart considers, the tongue articulates, the mouth finishes, the esophagus brings in all kinds of food, the windpipe gives sound, the lungs absorb all kinds of fluids, the liver causes anger, the gallbladder secretes a drop into it and calms it, the spleen laughs, the gizzard grinds, the stomach [causes] sleep, the nose [causes] wakefulness. (Berachos 61a; similarly in Midrash Vayikra Rabbah 4:4)
This is not an aggadic legend intended to be understood metaphorically. The descriptions of the functions of the tongue, mouth, esophagus, windpipe, lungs, stomach and nose are all clearly scientific descriptions intended to be interpreted literally. The account of the liver causing anger is also consistent with standard belief in the ancient world. Thus, the account of the function of the kidneys and heart are thus also clearly intended to be literal descriptions - and there is no important role ascribed to the brain. This, too, is consistent with standard Aristotelian belief in the ancient world. The Rishonim and Acharonim agree that Chazal were speaking literally, as discussed in my monograph, The Question of the Kidneys' Counsel. Elsewhere, the Gemara relates halachos pertaining to the kidneys of animal offerings to the kidneys' function in man of providing counsel. Other Midrashim likewise echo this understanding of the role of the various organs:
" 'And God said to Moshe: Pharaoh's heart has become heavy (kaveid)' - He was angry. Just as the liver is angry, so too the heart of this one became a liver (kaveid), without understanding, as a fool. (Midrash Shemos Rabbah)Like everyone else in the ancient world, Chazal thus likewise interpreted all Scriptural references to the heart (which most people today take as referring to the mind and thus the brain) literally. Scriptural references to the heart having various emotional states, to it housing wisdom and cognition, and to God judging a person based on examining his heart and kidneys, were all taken literally by Chazal.
"That is to say, the heart of Pharaoh was turned into a liver (kaveid) -- just as a liver has no understanding to understand and comprehend, so too there was no understanding in his heart to understand and comprehend. Therefore, his heart was hardened and was stubborn for him." (Midrash Lekach Tov)
2. Chazal were mistaken in this regard.
That should be self-evident. We now know that it is the brain that is used for all cognitive processes and for making decisions. The heart and kidneys have no such role. In fact, the heart can be replaced by an artificial pump, and the kidneys can be replaced by a dialysis machine. Doing this does not impair a person's mind in any detectable, significant way.
3. There is a fundamental connection between the mind/ free will, the soul, and the presence of a live person - and thus the mistaken belief that the heart and kidneys house the mind has fundamental ramifications on the question of determining death.
This is the part that some people were disputing. So I will explain it as clearly as possible, incorporating Dr. Stadlan's valuable observations.
The word "soul" is a vague term that has been interpreted differently by various figures over the ages. Some identified it as the intellect, some as consciousness, and some as a kind of metaphysical entity. The first two clearly relate to the mind. The third is essentially unspecified and incomprehensible, but as best as we can ascertain, it fundamentally relates to the mind, which is after all the part of us which absorbs Torah, exercises free will, and guides our actions. Jewish tradition has always maintained that the soul is involved in free will, and acquiring knowledge of God and Torah. Everyone agrees that these functions are accomplished in the brain. If one wants to claim that there are other aspects to the soul, which are housed elsewhere in the body, then what are they? Thus, as best as we can determine, the soul is housed in the mind. And the presence of a soul is certainly linked to the idea of an alive human being. Thus, not only is a functioning human brain the home of the mind, but if we are going to use the concept of the soul, then both the mind and the soul are housed in the brain.
There is further evidence that the soul is not housed elsewhere. The possibility of transplants and artificial organs sheds much light on the fundamental connection between the mind/ free will, the soul, and the presence of a live human being. In all such cases there are no halachic ramifications, such as murder, or transferring financial or other obligations from donor to recipient. One can replace someone's arm with a prosthetic, without being guilty of murder, and without changing the person's halachic status in any way. Clearly, then, the arm does not determine the presence of a person or of a soul. The same goes for many other body organs. Most significantly, one can do so with a heart. There are artificial hearts which have been successfully used for extended periods. Nobody would say that replacing someone's heart with an artificial heart means that you have killed them. This is further evidence that the soul resides in the mind - the one organ that cannot be replaced by an artifical substitute.
As Dr. Noam Stadlan points out, further evidence that the soul is housed in the brain comes from consideration of conjoined twins. "The only time halakhah or modern society even considers whether one or two humans have been born is when the newborn has more than one head. The duplication of every other organ (including the heart) does not raise any question of multiple identities or souls. Therefore, it appears that the universally accepted halakhah regarding issues of organ removal, substitution and transplantation, consciously or not, is based on the brain, and only the brain, being the seat of the soul."
We have to determine, which organ is it that fundamentally defines the presence of a person? Clearly it is the organ which houses the mind and thus the soul. Whichever organ that is, the presence and functioning of that organ will be the fundamental determinant of life. If the heart houses the mind, as Chazal thought, then if the heart is still beating, the person is still alive; if it stops, he is dead. But if it is the brain which houses the mind, then a person dies at brain death. Chazal lived in a world where a person's mind and soul were mistakenly thought to be housed in the heart and to thus be measured via a pulse and respiration, rather than to be housed in the brain, and thus to be measured via neural activity.
Now, it's certainly theoretically possible that there are additional factors which mean that a person is still rated as alive while the heart is beating. But nobody has proposed what they might be, or offered reason to believe that such factors even exist. Furthermore, since it is clear that a mistaken belief that the heart houses the mind would certainly cause one to define death based on the pulse and respiration, we know that Chazal's definition would have been, at least in part, based on an error.
We thus need to re-evaluate the determinant of death based on modern knowledge, rather than solely deducing it from Chazal. We know that a functioning human brain is the home of the mind, and it is the sole organ that needs to be present and cannot be replaced by an artificial substitute. For both of these reasons, we can deduce that the brain is the home of the soul and the sole determinant of life.
4. Even aside from all the above, the fact that maintaining a brain-dead person's circulation and respiration was impossible in Chazal's time, means that drawing inferences to today from Chazal's statements and rulings is fundamentally in error.
If Chazal say that a person is considered alive as long as he is breathing or has a heartbeat, it has to be taken into account that in their day, this was certainly the case, and there are no ramifications for how brain death is rated. Even with someone who considers brain death to be death, if he were to go back in time to Chazal's era and were to forget how to do CPR, he would agree that life and death, in that time, depends on pulse and respiration! So we cannot draw inferences from Chazal's statements about gauging life, which were entirely reasonable for their era, to the modern era.
To put it another way; even if Chazal were to theoretically have known about brain death and rated it as death, they would not have expressed their rulings any differently. So we cannot infer from their words that they did not rate brain death as death.
5. The problems involved in diverging from Chazal and reassessing the halachah are serious, but they have already been overcome in other contexts.
It's no small matter to re-evaluate halachah based on modern science. Some people justly fear that it means sliding towards Conservative Judaism. I myself follow Rav Herzog that in general, canonized halachah in the Talmud should not be changed even if based on scientific error, such as with Chazal's license to kill lice on Shabbos due to their mistaken belief that lice spontaneously generate.
Yet others do indeed say that we should change halachah based on our increased knowledge of science - such as Rav Lampronti in the case of lice. And even Rav Nissim Karelitz and Rav Elyashiv say that one should be stringent due to Rav Lampronti's view (sources to follow this week).
But even according to Rav Herzog's approach, this case is very different from that of lice, for two reasons. One is that we are not necessarily undermining the Talmud at all. The Talmud does not say "brain death is not death"! Rather, it makes some statements about determining the condition of a person buried under rubble, and other such statements, which were perfectly valid given the medical possibilities of the era. It can thus be presented as a case of nishtaneh hateva without having to bring in Chazal's misunderstanding of physiology.
Another difference is that if we accept brain death as death, then organ donation is viable and lives can be saved. It is thus a matter of pikuach nefesh, which always overrides everything. The Talmud gives a formal ruling that one can transgress Shabbos to save the life of a 7-month fetus but not that of an 8-month fetus, because (in Chazal's view) only the former was viable. Forget nishtaneh hateva; this was never true. And nobody follows Chazal's ruling in this.
Finally, we see that virtually all the Poskim today are willing to diverge from following Chazal in cases where they recognize that Chazal were scientifically mistaken and especially in cases of life and death. Virtually nobody follows Chazal in ruling on kidney transplants. Nobody follows Chazal, as explained by Chasam Sofer, that a person who is not currently breathing will never breathe again and is considered dead - everyone would do CPR. And nobody follows Chazal that an 8-month fetus is not viable. It is always presented in a (sometimes intellectually dishonest) way that avoids the idea that we are undermining Chazal, such as by invoking "nishtaneh hateva" even where that is clearly not the case. The same can be done here - and in fact, this really is a case of nishtaneh hateva - of medical possibilities.