The Mishna on today’s daf teaches:
One who says: ‘The offspring of this animal shall be a burnt offering and the animal itself shall be a peace offering,’ his words stand. But if he says first ‘the animal shall be a peace offering’ and then ‘and its young shall be a burnt offering,’ its young is regarded as the young of a peace offering. This is the teaching of Rabbi Meir. Rabbi Yosei says that if he intended to say this at first, since it is impossible to mention both kinds of sacrifices simultaneously, his words stand; but if after he already said intentionally ‘this shall be a peace offering,’ he changes his mind and says ‘its young shall be a burnt offering,’ its young is regarded as the young of a peace offering.
Rabbi Meir’s teaching is straightforward. Although someone has the ability to sanctify the embryo of a pregnant animal separately from its mother, that is only if he first sanctified the embryo. If, however, the mother is sanctified first, the embryo automatically receives its mother’s sanctity, and it cannot be sanctified separately. Rabbi Yosei argues that if the person’s intent was to declare different sanctifications for the mother and the embryo, he can do so even if he mentions the mother first. He cannot, however, change his mind once the statement is made.
Rav Pappa explains that Rav Yosei’s teaching is important in a case where the statements were made tokh ke-dei dibbur – within the span of an utterance. Generally speaking, Jewish law allows someone to change their mind and restate their intention if they do it within a short amount of time. Most rishonim explain that this is the amount of time that it takes to utter a statement welcoming one’s teacher – shalom aleikhim mori ve-rabbi. In the case of sanctified objects, however, this rule does not apply, and once a statement has been made it cannot be rescinded, even tokh ke-dei dibbur.