Chullin 100 - 106
The Blood Link
Eating the flesh taken from an animal while it is still alive is forbidden by the Torah passage that links it to the forbidden consumption of blood: "Only be sure not to eat the blood, for the blood is the life, and you may not eat the life with the meat." (Devarim 12:23)
The significance of this link in determining which forms of living things are covered by this ban is a matter of debate. Rabbi Elazar extended the ban to all cattle, beasts and fowl, both those considered pure and permitted for consumption after shechita and those considered impure and forbidden for consumption. His understanding is that since the Torah linked this ban to the ban on blood, it applies to every form of life whose blood is forbidden for consumption.
The other Sages, however, limited the ban on eating the flesh of an animal before its death to those species whose flesh is permitted after shechita. "You shall not eat the flesh along with the lifeblood", they understand, is the Torahs way of restricting this ban to flesh which may be consumed when the lifeblood has already been shed, thus eliminating those impure species whose flesh is forbidden even after shechita.
Should one eat the flesh taken from an non-kosher animal or fowl before shechita, he will be guilty of violating only one Torah prohibition according to these Sages, and two according to Rabbi Elazar. Rabbi Meir is even more restrictive, limiting the prohibition of eating flesh from a living creature to cattle alone. This position is based on the fact that the passage introducing this law (ibid. 12:2) speaks of slaughtering "of your herd and your flock."
This debate on how far the ban on eating the flesh from a living creature goes applies only to Jews for whom there is a distinction between kosher and non-kosher species. For the non-Jew obligated by the Noachide commandment "But flesh with its life, which is its blood, you shall not eat." (Bereishet 9:4) there is no such distinction, and he is prohibited to consume the flesh taken from any animal life while it is still alive.
- Chullin 102a
The Six-Hour Wait
In order to safeguard Jews against violating the Torah prohibition against eating meat cooked in milk our Sages instituted a ban against eating dairy products right after meat ones. The separation between the consumption of the two is defined by the Sage Mar Ukva as the time elapsing between one meal and another which the Shulchan Aruch (Yoreh Deah 89:1) rules is six hours.
Two different reasons are given by the commentaries for the length of time and for why it applies only to milk after meat and not vice versa. One is that the juices and flavor from meat linger for a long time after it is eaten. According to this reason, if one did not actually eat the meat but only chewed it to soften it for a baby, there is no need to wait before eating dairy. There is no need for concern that meat has lingered between the teeth, but if it is found there it must be removed even after six hours.
Rambam (Laws of Forbidden Foods 9:28), however, offers another reason based on a dialogue in our gemara. Rabbi Acha bar Yosef asked Rabbi Chisda about the status of meat lodged between ones teeth. His response was to refer him to a passage in the Torah (Bamidbar 11:33) that describes the Heavenly punishment that already befell the Jews for sinfully complaining about their lack of meat "while the meat was still between their teeth." This is interpreted as an indication that meat retains its status after being chewed and the six-hour waiting period is required because of our concern that meat may be lingering between the teeth, and time is needed for it to lose its status as meat.
According to this second approach one must wait six hours even if he only chewed food for a child. But if he found meat between his teeth after six hours, there would be no need to remove it because it has lost its status of meat. This leniency is ruled out by the first approach that interprets the response of Rabbi Chisda as extending the status of meat indefinitely to any particles between the teeth and limiting the impact of the six-hour wait to meat that has been swallowed with only the taste remaining.
The halachic ruling is to follow the stricter dimensions of both opinions to wait six hours (or the length of time according to varying customs) even after only chewing meat, and to remove meat from between the teeth even after six hours.
- Chullin 105a