One of the 39
melakhot in the
mishnah on 73a is
borer, “separating” a mixture of waste and food. This is one of the
melakhot in
siddura de-pat, the process of making bread.
In general, as the
rishonim learn from
Shabbat 74a–b, you may only select from a mixture under three conditions: (1) you remove the food from the waste, and not vice versa; (2) you remove the food by hand; (3) you remove it for immediate eating. When any of these conditions are missing, a penalty of
karet is at stake. Each condition is very detailed, and I don't plan to cover them here.
One of the major questions with
borer is how far its application goes. What kinds of mixed items are included in the prohibition? Just waste from food? Or even forks from knives?
At one extreme, there is the Talmud Yerushalmi,
Shabbat 7:2, which quotes Rabbi Yudan's limited view of
borer:
אמר רבי יודן יש שהוא בורר צרורות כל היום ואינו מתחייב יש שהוא נוטל כגרוגרת ומיד מתחייב היך עבידה היה יושב על גבי כרי וברר צרורות כל היום אינו מתחייב נטל לתוך ידו כגרוגרת ובירר חייב
It sounds like the only way to transgress
borer is to clear the very last stone from a pile of grain. You have to entirely purify the food from its waste. Unlike what happened
with zoreh, this is a case where you'd be happy for the Rema to pick up the Yerushalmi, at least if you like
kulot.
At the other extreme, we have the “great question” of Rabbi Yehiel Michel Epstein, in
Arukh ha-Shulkhan,
O"H 319:8. He assumes that
borer applies to literally “everything,” even a collection of non-food items. Were it indeed so, he objects, “we could not find our hands and feet!”
