daf bit: Zevachim 106

Background: the torah holds that there is a condition that I'll describe here as ritual impurity. This happens to everybody from time to time and there are standard remedies for reversing it. It's not trivial, but it is also not like having the plague, and often as not you didn't really even have any control over it. However, the torah is concerned with transmission of this ritual impurity, so there are some restrictions -- for example, a priest in this state cannot offer sacrifices.

The mishna says that one who is ritually impure and eats of any sacrifice, whether that meat was itself ritually pure or impure, is culpable. R. Yose the Galilean disagrees, saying that if the meat was ritually impure already then the person has done no harm. Finally, a person who is ritually pure who eats ritually impure meat is not culpable because you are only liable for what arises from your own state. (Ritual impurity is not the same thing as non-kosher.) (106a)

The talmud usually has many tangential conversations -- there you are in the middle of tort law and suddenly you're discussing the kashrut of an oven and how even a voice from heaven arguing for Rabbi Eleazar wasn't sufficient to change the law. But this tractate has been remarkably single-minded for 100+ pages. Wow.