Study Bava Kamma folio 46A with parallel Hebrew-English text, traditional commentary, and modern study tools. Free access to Babylonian Talmud online.
But if an ox is entirely rendered forewarned, you do not find an element of innocuousness with regard to it at all. If it is forewarned with regard to goring with either horn, its status of innocuousness is completely revoked.
§ The Mishnah teaches that R' Eliezer says: An ox has no sufficient safeguarding at all other than slaughtering it with a knife. Rabba said: What is the reason for the opinion of R' Eliezer? It is as the verse states with regard to a forewarned ox: “And the owner has not secured it” (Exodus 21:36)
Abaye said to Rabba: If that is so, does that which is written with regard to a pit: “And if a man shall open a pit, or if a man shall dig a pit and not cover it” (Exodus 21:33), also mean that once a pit has been dug the owner no longer has any adequate way of covering this pit, which would exempt
And if you would say that indeed that is the halakha, but didn’t we learn in a Mishnah (52a) that if he covered the pit appropriately, and an ox or a donkey fell into it and died, he is exempt? Evidently, a pit can be covered adequately.
Rather, Abaye rejected Rabba’s explanation of R' Eliezer’s opinion, and said that this is the reason for the opinion of R' Eliezer: As it is taught in a baraita that R' Natan says: From where is it derived that one may not raise a vicious dog in his house, and that one may not set up an unstable lad