Answer: He believed the brother because he spoke l'Fi Tumo (unaware of the consequences of his words):

(Rav Yehudah): A man said l'Fi Tumo 'I remember when I was a child... they took me out of school... and immersed me to eat Terumah';

R. Chiya says that he concluded 'my friends avoided touching me, and called me 'Yochanan who eats Chalah'.'

Rebbi established him to be a Kohen based on this.

(Beraisa - R. Shimon ben Elazar): Ma'aser Rishon establishes a man to be a Kohen, just like Terumah does. If he takes a portion in Beis Din, this does not establish him.

Question: Ma'aser Rishon is given to a Levi!

Answer: Our Tana holds like R. Elazar ben Azaryah:

(Beraisa - R. Akiva): We give Terumah to a Kohen and Ma'aser Rishon to a Levi;

R. Elazar ben Azaryah says, Ma'aser Rishon is given even to a Kohen.

Question: Even R. Elazar ben Azaryah agrees that Ma'aser may be given to a Levi! (How can we establish the man to be a Kohen?)

Answer: R. Elazar ben Azaryah holds that after Ezra fined the Leviyim (for not returning to Eretz Yisrael), Ma'aser Rishon is given only to a Kohen.

Question: Perhaps it happened to be given to a Levi!

Answer (Rav Chisda): We knew that Yochanan's father was a Kohen. There was a rumor that Yochanan was Pasul for Kehunah because his mother was divorced or did Chalitzah. People saw Yochanan receiving Ma'aser Rishon.

He is certainly not a Levi! If he was truly a Pasul Kohen, he would not receive Ma'aser.

Even the opinion that permits anyone to eat Ma'aser Rishon agrees that it is distributed only to a Levi or Kohen.

(Beraisa): If he takes a portion in Beis Din, this does not establish him.

Question: If distribution in Beis Din does not establish him, what does?!

Answer (Rav Sheshes): The Beraisa means: one who receives a share of Terumah with his brothers from his father's estate is not established to be a Kohen.

Question: This is obvious!

Answer: One might have thought that just like the others receive Terumah to eat, also this son;

Rather, it is possible that he receives Terumah to sell it.

(Mishnah - R. Yehudah): We do not establish Ploni to be a Kohen based on one witness... (R. Elazar says, when there are no protests, one witness suffices. R. Shimon ben Gamliel says, we establish a Kohen through one witness.)

Question: R. Shimon ben Gamliel says the same thing as R. Eliezer!

Suggestion: Perhaps R. Eliezer holds that even one witness can protest, and R. Shimon ben Gamliel requires two.

Rejection: R. Yochanan taught that all agree that a protest requires at least two!

Answer #1: The case is, we know that Ploni's father is a Kohen. There was a rumor that Ploni is Pasul (due to his mother), so we demoted him from Kehunah. One witness testified that he is a proper Kohen, and we reinstated him;

Two witnesses came and said that he is Pasul, so we demoted him. Now, a second witness testified that he is a proper Kohen;

All agree that we join witnesses who did not testify together. The question is whether or not we are concerned for disgrace to Beis Din.

The first Tana (R. Eliezer) does not allow a disgrace of Beis Din. Since we demoted him, we do not reinstate him;

R. Shimon ben Gamliel says that we reinstate him. We are not concerned for the disgrace of Beis Din.

Objection (Rav Ashi): If so, they would argue even if both witnesses that he is a Kohen came after the two who protested!

Answer #2 (Rav Ashi): They argue about whether or not we join witnesses who testified separately;

Other Tana'im (R. Nasan and Chachamim) also argue about this:

(Beraisa): Two witnesses do not join unless they saw the testimony together;

R. Yehoshua ben Korchah says, they join even if they saw the testimony at different times.

Two witnesses join only if they testify together;

R. Nasan says, one can testify today and the other on another day.

(Mishnah): If Nochrim took a woman captive for money (they intend to charge ransom to release her), she is permitted to her husband;

If she was taken for a capital offense, she is forbidden to her husband.

(Gemara - Rav Shmuel bar Rav Yitzchak): This applies only when Yisrael are in control. If the Nochrim are in control, even when taken for money she is forbidden to her husband.

Version #1 - Question (Rava - Mishnah): R. Yosi ha'Kohen and R. Zecharyah ben ha'Katzav testified about a girl who was taken for collateral for a loan in Ashkelon. Her family distanced themselves from her, even though the witnesses about her abduction said that she was never secluded.

Chachamim: If you believe the witnesses that she was taken for collateral, believe them that she was not secluded! If you do not believe them that she was not secluded, do not believe them that she was abducted!

Suggestion: Nochrim are in control in Ashkelon, and it was a problem only because she was taken for collateral. Had she been taken merely for ransom, she would be permitted (even without witnesses that there was no seclusion)!