Nedarim 19A

כל שכן דלא מעייל לספיקא.

holds that all the more so, one does not enter himself into uncertainty either. Therefore, uncertainty with regard to naziriteship is treated leniently.

אמר ליה אביי: במאי אוקימתא לספק נזירות להקל ⁠— כרבי אליעזר, אימא סיפא: ספק בכורות, אחד בכורי אדם ואחד בכורי בהמה, בין טמאה בין טהורה ⁠— המוציא מחבירו עליו הראיה.

Abaye said to R' Zeira: In what manner did you establish the Mishnah that states that uncertainty with regard to naziriteship is treated leniently? You established it in accordance with the opinion of R' Eliezer. Say the latter clause of that Mishnah: If there is uncertainty with regard to firstborns, whether human firstborns, or animal firstborns, whether non-kosher firstborns, i.e., the firstborn of a donkey, or the firstborn of kosher animals, the burden of proof rests upon the claimant. In other words, the priest cannot take the redemption money from the father of the child, or the animal from its owner, and conversely, if the father or owner mistakenly gave it to a priest, he does not get it back.

ותני עלה: ואסורים בגיזה ועבודה!

And it is taught in a baraita in that regard: But with regard to shearing and working these uncertain animal firstborns, they are forbidden, just like definite firstborns. This indicates a difference between the monetary issue, with regard to which it is ruled that the animal cannot be taken from the owner by the priest, and the prohibition, which applies despite the uncertainty. Evidently, even the tanna of this Mishnah does not hold that all uncertainties with regard to consecration are to be treated leniently.

אמר ליה: אמאי קא מדמית קדושה הבאה מאליה, לקדושה הבאה בידי אדם?

R' Zeira said to Abaye: This is not difficult. Why do you compare sanctity that emerges by itself, i.e., the sanctity of a firstborn, which results from objective reality and not human intent, to sanctity that emerges by the volition of a person and is dependent on his intention? Only with regard to the latter type of sanctity can it be established that a person does not intend to consecrate an item in an uncertain manner.