Keritot 19A

רבי שמעון פוטר בזו. רבי שמעון בן יהודה פוטר בכולן משום רבי שמעון.

The baraita continues by citing a dispute concerning R' Shimon’s opinion. R' Shimon exempts him from the obligation to bring an offering in this particular case. Since he became pure in between, at no point did he have definite knowledge that he was impure. For one to be liable to bring an offering for entering the Temple in a state of ritual impurity he must have knowledge of the impurity at the beginning and the end, and a lack of knowledge in the middle. R' Shimon ben Yehuda deems him exempt from the obligation to bring an offering in all of those cases, and he reports this opinion in the name of R' Shimon.

ואפילו בקמייתא? אמר רבא: הכא במאי עסקינן? כגון שהלך בראשון ובשעת הילוכו בשני שכח שהלך בראשון, ובהא פליגי, תנא קמא סבר: מקצת ידיעה כידיעה דמיא, ורבי שמעון סבר: מקצת ידיעה לאו כידיעה דמיא.

Before explaining the contradictions between the opinions of R' Yoḥanan and Reish Lakish, the Talmud asks a question about R' Shimon ben Yehuda’s statement: Can it be that R' Shimon holds that one is exempt even in the first case? Since he walked down both paths prior to entering the Temple, he had definite knowledge of his impure status. Rava said: What are we dealing with here? We are dealing with a case where he walked on the first path, and at the time of his walking on the second path he forgot that he had previously walked on the first path. Consequently, the knowledge that would render him liable to bring a sin offering is incomplete. And it is with regard to this point that they disagree: The first tanna holds that partial knowledge is considered like full knowledge, and R' Shimon holds that partial knowledge is not considered like full knowledge.