drawing the line when one can speak and when to remain silent.
hearing something from others can be equivalent to seeing with the eye, when there is a continuous line of reliable teachers and students, and no women.
but for R. Yohanan. the dividing line is that one can only speak when you truly know something. like you know that sleeping with your sister, your very beautiful sister, is forbidden.
(in the discussion about if one can squeeze fish for brine, R. Dimi defends his quotation from Rav citing a reliable chain and says that he saw it with his eye. there is a debate if hearsay testimony is permitted in the case of being a witness that a woman's husband is dead and she can remarry. or perhaps that is the only case when a woman's testimony is valid. Later in the page, R. Yohanan overhears two rabbis discussing differences between Bavel and Israel. He rebukes them for their ignorance by citing that should only speak when the matter is obvious, as obvious as sleeping with your sister is forbidden. we know from Bava Metzia that R. Yohanan thought his sister was very beautiful, more beautiful than him... for english translation of this talmud page, see here)