How can the gemoro prove that yisuvei da'ata must be less than a k'beitza (since a k'beitza is already a shiur sviah(?
It is clear from the sugya that yisuvei da'ata is a deoraysa. However, as is clear from the 3rd perek of Brachos (Amru malachei hashares....ad kezayis ad k'beitza), a k'beitza is only a shiur sviah derabanan. So perhaps we can challenge the gemoros proof: that it's true on a derabanan level the yisuvei da'ata must be less than a k'beitza (which is sviah derabanan); but perhaps the chiuv of yom kippur i.e. yisuvei da'ata deoraysa could perhaps be larger than a k'beitza. So what's the gemorro's proof.
Please help me out in this matter.
Thank you.
A.L., AUSTRALIA
The Ramban in Milchamos, Berachos 20a proves from your question that in fact the Machlokes between Rebbi Meir and Rebbi Yehudah as to whether a person must eat a k'Zayis or a k'Beitzah in order to be Chayav be'Virchas ha'Mazon is a Machlokes in Din Torah. The Ramban holds that there is a Machlokes between the Sugyos and that we do not hold like the Drasha of Rav Avira in Berachos 20b. This is also the opinion of the Ra'avad. However the Ba'al ha'Ma'or (ibid.) holds that k'Zayis and k'Beitzah are both d'Rabanan and mid'Oraisa we need a Shi'ur Seviyah. This is also the opinion of Rashi (Berachos 20b) and Rosh (7:24). Your question, therefore, remains according to these Rishonim.
Perhaps we can answer that since Rebbi Yehudah is of the opinion that a k'Beitzah must be eaten even to be Chayav mid'Rabanan, and he gives the reason that this is considered a Shi'ur Seviyah, even though it is not a degree of Seviyah to be considered a Seviyah Min ha'Torah, since at least mid'Rabanan it is considered a Shi'ur Seviyah, the Gemara in Yoma can prove that it must be more than Yesuvei Da'ata. This is because, if there was not some degree of Seviyah involved in eating a k'Zayis, Rebbi Yehudah could not even call it Seviyah d'Rabanan. The Gemara in Yoma hence proves that if a k'Beitzah is at least a bit satisfying (enough to be considered Seviyah mid'Rabanan) it is certainly more than enough to be m'Yasiv Da'as.
See He'oros from Rav Eliyashiv's Shi'urim on Yoma for an approach according to some opinions.
Dov Freedman