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The first Mishnah of the third chapter deals with a person who purchased an animal from a non-Jew and does not know whether it had previously given birth and was thereby exempted from the laws of the firstborn. The Mishnah specifically discusses one who purchases from a non-Jew, while the Gemara in our sugya records a dispute among the Amoraim regarding someone who purchased an animal from a Jew:
“Rav said: it is definitely a firstborn… Shmuel said: it is a doubtful firstborn… and Rabbi Yoḥanan said: it is definitely ordinary, non-sacred property.”
All three Amoraim attempt to assess the presumed intention of the seller. Rav maintains that the seller’s silence proves with certainty that the present offspring is a firstborn. Shmuel holds that his silence proves nothing conclusively and may be interpreted in different ways. Rabbi Yoḥanan, by contrast, maintains that the seller’s silence is clear evidence that the animal had already given birth previously. Rabbi Yoḥanan explains his position with the principle:
“Since there is a prohibition involved, he certainly would have informed him.”
investigation falls instead upon the purchaser rather than the seller.
According to Tosafot, the position of Rav and Shmuel is grounded in the prohibition:
“You shall not place a stumbling block before the blind” (lifnei iver lo titten mikhshol) to which we explore its varying applications.