The Jewish Legal System: Laws of Damages: Treatment of the Dying:
When is a person a TEREFAH? Is it permissible to kill such a person,or a dying person?
Maimonides (Laws of Murder and Preservation of Life, 2:7) defines a
Terefah as someone who, according to qualified medical opinion, suffers from a condition that will be fatal (later codifiers add: in a year or less); and he rules that one cannot be punished by the courts for killing such a person. However, such a killing is still ethically murder (ibid. 2:2); and if a person is not a Terefah, even if he is about to die of natural causes, killing him is definitely murder (see 2:7). There are grounds in Jewish law that permit removal of things that are artificially keeping a dying person alive, particularly if the person is unconscious and in pain; see Yoreh Deah 339:1 and later codifiers. But decisions about such matters should only be made after consultation with qualified Rabbis.
Here is a very general guideline: Nourishment (food, water, oxygen)
must not be withheld, but medications and artificial means of prolonging
life can be discontinued if the patient requests it or is no longer
conscious, and the doctors agree that there is no hope for recovery.
This guideline applies only to a terminal patient; brain death, or
being in a vegetative state, don't themselves justify withholding
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