Brain Death - Rav Bleich

 https://www.sefaria.org/Contemporary_Halakhic_Problems%2C_Vol_IV%2C_Chapter_XIII_Of_Cerebral%2C_Respiratory_and_Cardiac_Death.1?lang=bi&with=all&lang2=en

Chapter XIII Of Cerebral, Respiratory and Cardiac Death

The conflict between authentic Jewish teaching and societal espousal of so-called "brain death" criteria involves no scientific or factual controversy whatsoever. It does involve disparate views regarding the sanctity of human life, regardless of its quality, and conflicting perceptions of duties owed to the moribund patient.

Judaism regards every life as being endowed with infinite value; Judaism also regards every moment of life, regardless of its quality, as endowed with infinite value. Until all vital forces ebb from the body, as evidenced by total cessation of both respiratory and cardiac activity, human life must be treasured as a sacred gift. The adamancy of halakhic authorities in their refusal to accept "brain death" criteria is not at all an instance of other-worldly patriarchal figures refusing to acknowledge demonstrable scientific verities; it is entirely a matter of insistence upon the sanctity of every moment of human life. 

A person unfamiliar with the extensive rabbinic literature concerning this topic may well ask whether Judaism cannot accommodate a neurological definition of death. Support for such a position might be adduced from a superficial reading of the Mishnah, Oholot 1:6: "And likewise cattle and wild beasts … if their heads have been severed, they are unclean [as carrion] even if they move convulsively like the tail of a newt (or lizard) that twitches spasmodically [after being severed from the body]." Destruction of tissue as the result of liquefaction, it may be argued, is tantamount to severance or excision of such tissue. Consequently, there is indeed a measure of cogency in the argument that total liquefaction of brain tissue is tantamount to physiological decapitation. 

For halakhic purposes, dysfunction of an organ is not the equivalent of its destruction or excision. A male whose testes have been removed is forbidden to cohabit with a Jewess of legitimate birth; a person whose testes remain intact but have been rendered dysfunctional suffers no such liability. Similarly, an animal whose liver has been removed is a treifah and its meat is forbidden; the meat of an animal whose liver performs no physiological function is permissible. Excision is defined as removal, either as a result of trauma or surgical procedure. Alternatively, it is defined as degeneration of tissue either through necrosis to the degree that it becomes "tissue which crumbles in the finger" (basar she-nifrakh be-ziporen) or through "decay" to the degree that it becomes "tissue which a physician scrapes away" (basar she-ha-rofeh gorero), e.g., gangrenous tissue. The brain tissue of a patient pronounced dead on the basis of neurological criteria does not match, or even approximate, those levels of degeneration.