Siman 563

Our teacher, the Beit Yosef, wrote: One who meditated in his heart, not during the time of the mincha prayer, to fast tomorrow—it is not a kabbala, for it is no better than if he expressed with his mouth to fast tomorrow, which is not a kabbalat ta'anit, since he did not accept it during the mincha prayer. Thus far his words. And I do not know what he is teaching us, for he already explained this in the previous siman, section 6, where he wrote there: When does he accept it? During the mincha prayer… and if he did not express it with his mouth but meditated in his heart—it is a kabbala, provided that he meditates so during the time of the mincha prayer. Thus far his words, and what did he innovate here? And it is difficult to say that because the Tur wrote this law in this siman, he was also forced to write it so as not to skip the siman, even though he already wrote it. And it requires further study.

And we have already investigated in the previous siman, section 19, if one did not accept [the fast] while it was yet day during mincha: whether it is forbidden for him to eat, though it does not count for him as a fast, or whether it is completely permitted to eat. And the Mordechai holds so. And there in section 20 we proved that our teacher, the Rema, is in doubt, for according to the opinion of the Rambam it is forbidden, see there. Therefore, I say that all this applies to an oral acceptance not at the time of mincha, but if he meditated in his heart not at the time of mincha — he is permitted to eat according to all opinions. For the reason that we forbid eating there is because of a neder, as according to the system of the Rambam, a neder and a fast are the same matter, as he wrote there. And if so, regarding a neder we hold in Yoreh Deah siman 210 that one must specifically express it with his lips similar to an oath, as it is written: "to express with the lips." And if so, it is simple that it is nothing. (And so appears the opinion of the Magen Avraham, see there. And perhaps because of this the Beit Yosef wrote it here. But if so, why did he leave the matters unexplained?)