Allowing the forbidden for the sake of a Mitzvah

Question

There are a few mitzvot which are effected through the use of that which is specifically forbidden. The first two examples are of tzitzit being made from shatnez and the mei sotah being infused with the ink from erasing the shem hashem. In an associated sense, we are mikayem the mitzvah of matzah through the use of a flour/water mixture which would be otherwise forbidden on pesach (though it is not precisely parallel). An even further extension might be that a kohen gadol or a nazir can be metameh himself for a met mitzvah (Mishna Nazir 7:1, "A Kohen Gadol and a nazir do not render themselves ritually impure for their relatives, but they may become impure for a "met mitzva"") And as a logical end, we have that pikuach nefesh pushes off everything and safek nefashos pushes off shabbos. But I am looking more at the specific mitzvot which override specific laws, not just ones which allow us to push off 'all' that is forbidden.

Are there other specific mizvot which are fulfilled through an action or a thing which would otherwise be forbidden?

Answer 1

The best example is Yibum, in which the brother of a dead husband (died without children) marries his widow. It is normally forbidden to marry one's sister-in-law. Another example is the belt of the Priestly Vestments which is made of both wool and linen, meaning that it is sha'atnez (according to some opinions). Furthermore, an example would be the mitzva of Metzora (person with "leprosy") shaving his head at the completion thereof. This would normally be in violation of the commandment to not round out the corners of one's head.

As an aside, there is no prohibition against flour/water mixtures on Pesach, only against leavened dough and bread.

There is another category of commandments, which become permissible when you do them but were prohibited up until that point. The best example of this is the קרבן עומר, the first sacrifice brought from new grain (brought on the 16th of Nisan). Before the sacrifice, חדש is prohibited. Afterward it is permitted. The sacrifice itself is what makes it permissible.

There is much more to be said on this topic, but we don't have the time/space for several שיעורים on מסכת יבמות. If you would like to look into it, the places to start are .יבמות ג:–ח and .נזיר לט:–מב and .מנחות ה:–ו. There are also a significant amount of ראשנים and אחרונים on this very subject.

Thank you to Seth J for pointing out what is left out of my answer. The most famous case in which a mitzva pushes off only some of a prohibition is the prohibition of טומאה in the בית המקדש, and the mitzva that all קרבנות need to be done on time. In the case that there is unavoidable טומאה at hand, the rule is טומאה דחויה בציבור, i.e. the impurity is pushed off, but it is not permitted (known as טומאה הותרה בציבור). This means that we do what needs to be done even though the כהנים or the כלים are טמאים, but we are careful not to do anything extra. The other side of this argument would say that once we are letting טומאה into the מקדש, its all מותר.