Generally speaking, only healthy animals can be slaughtered for kosher food. Thus, a tereifah – an animal that has a terminal condition – cannot be used. Nevertheless, if an animal is merely a mesukenet – it is ill – and its owner wants to slaughter it so that he can benefit from its meat, the Mishnah teaches that such shehitah would be kosher, assuming that the animal shows a sign of vitality when killed. According to Rabban Shimon ben Gamliel this would require movement of the animal’s limbs; according to Rabbi Eliezer, it is enough if blood spurted out at the time of shehitah.
In searching for a source for this ruling, one suggestion raised by the Gemara is that it is based on a passage in SeferYehezkel (4:14) where we find this statement of the navi:
Then said I, ‘Ah Lord God! behold my soul has not been polluted, for from my youth up even until now have I not eaten of that which died of itself [neveilah], or is torn of beasts [treifah]; neither came there abhorred flesh into my mouth.’
This statement was made after God commanded Yehezkel to eat disgusting things, and the prophet’s response was to object that he had always been careful to avoid anything that was not pure and untainted. The simple explanation is that Yehezkel was arguing that as a kohen he had to be even more careful than ordinary Jews regarding the food that he ate, as neveilah and treifah are not only unkosher, but ritually impure, as well. Nevertheless, the Sages of the Gemara understood that the prophet could not possibly have prided himself in simply keeping the straightforward halakhah. They therefore interpreted the passage as follows:
‘Behold my soul has not been polluted,’ for I did not allow impure thoughts to enter my mind during the day so as to lead to pollution at night.
‘For from my youth up even till now have I not eaten of neveilah or treifah,’ for I have never eaten of the flesh of an animal concerning which it had been exclaimed: ‘Slaughter it! Slaughter it!’ – i.e., the flesh of a dying animal, which was slaughtered with haste before it died.
‘Neither came there abhorred flesh into my mouth,’ for I did not eat the flesh of an animal which a Sage pronounced to be permitted.
Thus we find that avoiding such meat is a stringency that Yehezkel kept, but that others are not obligated to keep.