cellio | daf bit: Shevuot 9

The g'mara is discussing the mishna about which offerings atone for different kinds of sins involving tumah, ritual impurity. In the case where there is no knowledge either before or after, i.e. the person who sinned has no clue that he did anything wrong, the rabbis discuss how we know that the Yom Kippur, new moon, and festival goats atone. All of them are called sin-offerings, but that doesn't necessarily mean these particular sins. Previous discussion explained Yom Kippur. In arguing for the new moon, R' Yehudah says we know this because the text says "a sin offering unto the Lord", which means a sin known only to the Lord, meaning not known to the person who erred. (There's a lot of discussion about the choice of preposition.) The g'mara then derives from textual similarities that the festival goats also atone for this category of sin. (9a-b)