Chapter 3, Mishna 8
It appears in the Mishna about a dually conditional eruv that Rabbi Yehuda accepts the principle of retroactive designation, but elsewhere he seems to reject it. Is the Mishna wrong?! Several cases are brought to assess this question: Tithing wine by verbal designation rather than actual separation. Commingled bird offerings of two women who gave birth. A pious person sent by an ignoramus to purchase produce from another ignoramus. An eruv established for the benefit of only one person in a group, and the choice to be made later. An eruv established for the whole year that takes hold on a particular Shabbos only if the person actualizes the intent to do so. Can he do so after Shabbos begins?