Tosfos in Sukkah asks the question of why we sit in the succah on shemini atzeres but do not take arbah minim, he answers that the reason is because sitting in the sukkah is not viewed as a strange action during that time in the year however, arbah minim would be and therefore it would be an insult to the yom tov of shemini atzeres.
My question is why couldn't Tos' just answer that we sit in the sukkah on shemini atzeres because of safek deoraysa lechumra and we don't take arbah minim because the chiyuv min hatorah is only bayom harishon?
Yitzi Loewy, USA
The Ran (on the Rif, 22b DH v'Hilchasah) gives your answer, as does Rav Simchah from Desau in his glosses (printed at the back of most Gemaras). The Aruch la'Ner explains that Tosfos did not want to rely on this answer because, as far as Yom Tov Sheni is concerned, Chazal were Machmir even in cases of Safek d'Rabanan; for example we eat Maror on the second night of Pesach (and make a Berachah on it). The Ritva says that this is so that people should not come to be Mezalzel Yom Tov Sheni.
Dov Freedman