(Mishnah) (R. Elazar): A person does not need to declare part of Demai to be Maaser Ani.

(Chachamim): He must declare it but need not separate it.

If a person declared Terumas Maaser and Maaser Ani of Demai, he should not separate them on Shabbos. If a Kohen or a poor man regularly eats with him, they may eat, as long as he informs them.

(Gemara) (R. Ba bar Huna citing Rav): If one eats his fruit that had not had Maaser Sheni separated, he is liable to death (from Heaven).

Question: Why does R. Elazar say that one does not need to declare Maaser Ani? Since the seller knows that it involves death (from Heaven) he will separate it.

Why do Chachamim (Rabbanan) require declaring but not separating? Even if we are stringent to require him to declare Maaser Ani, he does not need to actually separate it and he may keep it for himself, so why not require him to do such a minor thing in order to avoid doubt!

Baraisa (R. Eliezer): If they saw him separating Maaser Sheni (from his produce, can he be assumed to have separated Maaser Rishon)? He is believed to have separated Maaser Rishon.

(Chachamim): One who is believed for Rishon is believed for Sheni; but one who is believed for Sheni isn't believed for Rishon.

Question: What is R. Eliezer's reasoning? Is it that he isn't suspected of separating Sheni before Rishon or simply because if they see him separating Sheni, we can also assume that he separated Rishon (as there is no difference between them)?

Answer: Didn't our Mishnah say that R. Eliezer says that a person does not need to declare part of Demai to be Maaser Ani. This means that he does need to declare Maaser Rishon? If the owner separated Sheni, where he doesn't have Tovas Hana'ah (benefit of gratitude of the recipient that the owner choose to give it to) (since the owner actually keeps it entirely for himself), but nevertheless, we assume he separated Rishon; if he separated Maaser Ani, that the owner only has Tovas Hana'ah (as he must give it over to the poor of his choice, but he doesn't keep it all for himself), certainly if he separated Maaser Ani, we can assume that he separated Maaser Rishon! (This shows that the reasoning of R. Eliezer is that if he separated Sheni, he certainly separated Rishon.)