[If] one slits the ear of a bechor, it may never be slaughtered; [these are] the words of R’ Eliezer. The Sages [however,] say: When it develops another blemish, he may slaughter it on its account. It happened with an old ram with dangling hair [that] a questor spotted it. He said, “What is the reason for this?” They said to him, “It is a bechor, and it may not be slaughtered unless it has a blemish”; [whereupon] he took a dagger and slit its ear. The matter came before the Sages, and they permitted it. [When] he saw that they had permitted [it], he went and split the ears of other bechoros, and they prohibited [it]. Once children were playing in the field, and they tied the tail of lambs one to another. The tail of one of them was torn off, and it was found to be a bechor. The matter came before the Sages, and they permitted it. [When] they saw that they had permitted [it], they went and tied together the tails of other bechoros, and they prohibited [it]. This is a rule: Whatever is [done] with knowledge is prohibited, and without knowledge is permissible.