Pesachim 16A

ספק משקין ליטמא ⁠— טמא. לטמא אחרים ⁠— טהור, דברי רבי מאיר. וכן היה רבי אלעזר אומר כדבריו. רבי יהודה אומר: לכל טמא.

If there is uncertainty whether or not a certain liquid has become ritually impure, it is presumed impure. It is an uncertainty with regard to Torah law, and the halakha is stringent in such cases. However, if the uncertainty is with regard to rendering other items impure, they are pure, as liquids transmit impurity by rabbinic law, and the halakha is lenient with regard to uncertainties of that kind. These are the statements of R' Meir, and R' Elazar would say likewise in accordance with his statements. R' Yehuda says: When there is uncertainty with regard to these liquids, the item is impure in all cases, even in terms of transmitting impurity to other items, as he maintains that the impurity of liquids is by Torah law.

רבי יוסי ורבי שמעון אומרים: לאוכלין טמאין, לכלים טהורין.

R' Yosei and R' Shimon say: In cases of uncertainty as to whether or not these liquids transmitted impurity to foods, the ruling is that the foods are impure, in accordance with the principle that when there is uncertainty in cases of Torah law, the halakha is stringent. R' Yosei and R' Shimon maintain that liquids transmit impurity to food by Torah law. However, when there is uncertainty as to whether or not these liquids transmitted impurity to vessels, the halakha is lenient, and they are pure. Even R' Yosei and R' Shimon concede that liquids transmit impurity to vessels only by rabbinic law. This baraita clearly indicates that R' Meir and R' Yosei indeed dispute whether or not the impurity of liquids applies by Torah law.