Nedarim 19B

הא מכירין ⁠— אסורין!

but where they are familiar, it is forbidden, even if the person mentioned teruma without specification, and there is still uncertainty with regard to which teruma he was referring.

אמר רבא: גבי כרי קסבר כל שספיקו חמור מודאי ⁠— לא מעייל נפשיה לספיקא, דאילו גבי נזיר ודאי, מגלח ומביא קרבן ונאכל. על ספיקו, לא מצי מגלח.

Rava said that the contradiction can be resolved as follows: With regard to the case of a vow conditioned on the volume of a heap, R' Yehuda holds that wherever uncertainty is more stringent than certainty, one does not enter himself into a state of uncertainty. Uncertain naziriteship is more stringent than definite naziriteship, as while a definite nazirite has a remedy, i.e., at the end of his naziriteship he shaves his hair and brings an offering and it is eaten, one cannot shave his hair for uncertain naziriteship. He cannot bring an offering in case he is not a nazirite, which would render his offering a non-sacred animal in the Temple courtyard. Since he cannot bring an offering he may not shave.

אמר ליה רב הונא בר יהודה לרבא: אמר ״הריני נזיר עולם״, מאי?

Rav Huna bar Yehuda said to Rava: According to your answer, that R' Yehuda holds that he is not a nazirite only because uncertain naziriteship is more stringent than definite naziriteship, if one said: If there are 100 kor in this heap I am hereby a permanent nazirite, what is the halakha? In this case, uncertainty is apparently not more stringent than certainty, as the naziriteship will never end even if it is definite.

אמר ליה: נזיר עולם נמי ספיקו חמור מודאי, דאילו ודאי הכביד שערו ⁠— מיקל בתער, ומביא שלוש בהמות. ואילו ספיקו ⁠— לא.

Rava said to him: Even with regard to a permanent nazirite, his uncertainty is more stringent than certainty with regard to a different halakha: As when the hair of a definite nazirite is too heavy for him he may lighten the hair with a razor and bring the 3 animal offerings that a nazirite brings when he has completed his term of naziriteship, before continuing to observe naziriteship; whereas in a case of uncertainty he cannot do so. Since it is not certain that he is a nazirite, he cannot bring these offerings and is therefore prohibited from shaving. Therefore, uncertainty is more stringent than certainty with regard to a permanent nazirite as well.