Love Conquers Fear
Reish Lakish (Rabbi Shimon ben Lakish) said, “Any time you find a conflict between a ‘positive commandment’ and a ‘negative commandment’, if you are able to fulfill both of them — good! But if you cannot fulfill them both, then the ‘positive commandment’ comes and pushes aside the ‘negative commandment’.”
According to this, the only time a positive command “overrides” a prohibition is if the command cannot be fulfilled in a manner that does not violate the prohibition. An example discussed in our sugya is the mitzvah to shave the entire head of a metzora. Since it can be done with tweezers and without a razor, the mitzvah does not grant permission to use a razor, which would violate the prohibition against shaving, since the mitzvah can be done without a prohibited razor. A modern-day example would seem to be to not allow linen tzizit on a wool garment, since wool could be used without pushing aside the prohibition against wearing shatnez.
Clean But Not a Haircut
“A nazir may wash his hair with shampoo or untangle his hair with his fingers, but may not comb his hair.”
One of the prohibitions for a nazir is not to cut his hair. The gemara on our daf explains that when he shampoos or separates the hair he does not intend to remove hair, but when he uses a comb he intends to remove (cut) loose hairs (Rashi). A question raised by commentaries is why the reason for not using a comb is not given as “psik reisha” — certain to happen — and some hair will come out even if he does not intend to remove any hair (Rosh, Rashi on Shabbat 50b).