Whether a Gentile May Perform a Valid Circumcision

(Genesis 17:8-9) It was stated: From where do we know that a circumcision performed by a gentile is invalid? Dari bar Pappa in the name of Rav said: from "And you shall keep My covenant" [the duty falls on one who is himself in the covenant]. Rabbi Yohanan said: from (verse 13) "he must surely be circumcised" [Hebrew himmol yimmol, one who is himself circumcised circumcises others]. What is the difference between them? There is a difference regarding a woman. According to the one who derives it from "And you shall keep My covenant," a woman is excluded, since a woman is not subject to circumcision. But according to the one who derives it from "he must surely be circumcised," she is included, since a woman is regarded as one already circumcised. But is there anyone who holds that a woman is not subject to circumcision? Is it not written, "And Zipporah took a flint" [and circumcised her son]? Read it instead as "And she had taken." But it is written, "and she cut off" the foreskin! Read it instead as "and she had it cut off" [by another].