Berakhot 15B

דילמא רבי יהודה היא, ולכתחלה הוא דלא, הא דיעבד ⁠— שפיר דמי.

Perhaps it is in accordance with the opinion of R' Yehuda, and although ab initio a deaf-mute may not read, after the fact his reading is valid?

לא סלקא דעתך, דקתני, חרש דומיא דשוטה וקטן. מה שוטה וקטן דיעבד נמי לא ⁠— אף חרש דיעבד נמי לא.

The Talmud responds: This could not enter your mind, as it was taught: A deaf-mute, an imbecile, and a minor, in one phrase in our Mishnah; to teach that a deaf-mute is similar to an imbecile and a minor. Just as in the case of an imbecile and a minor, even after the fact, their reading is not valid, so too the reading of a deaf-mute, even after the fact, is not valid.

ודילמא הא כדאיתא והא כדאיתא.

The Talmud rejects this assertion based on their appearance in a common list: And perhaps this case is as it is and that case is as it is; although listed together, the circumstances of each case may be different, and no definite proof can be drawn from their juxtaposition.

ומי מצית לאוקמה כרבי יהודה? והא מדקתני סיפא: רבי יהודה מכשיר בקטן, מכלל דרישא לאו רבי יהודה היא?!

The Talmud objects from a different perspective: And can you really establish that the first clause in the Mishnah is in accordance with the opinion of R' Yehuda? But from what was taught in the latter clause of the Mishnah: And R' Yehuda deems a minor fit to read the Megilla, this proves by inference that the beginning of the Mishnah is not in accordance with the opinion of R' Yehuda?

ודילמא כולה רבי יהודה היא ותרי גווני קטן, וחסורי מחסרא, והכי קתני: הכל כשרין לקרות את המגילה, חוץ מחרש שוטה וקטן. במה דברים אמורים ⁠— בקטן שלא הגיע לחנוך, אבל קטן שהגיע לחנוך ⁠— אפילו לכתחלה כשר, דברי רבי יהודה, שרבי יהודה מכשיר בקטן.

The Talmud rejects this challenge as well: And perhaps the entire Mishnah is in accordance with the opinion of R' Yehuda, and there are two types of minors, and the Mishnah is incomplete and it teaches as follows: All are fit to read the Megilla except a deaf-mute, an imbecile, and a minor. In what case is this said? With regard to a minor who has not yet reached the age of training to fulfill the mitzvot. However in the case of a minor who has reached the age of training, even ab initio he is fit to read the Megilla. This is the statement of R' Yehuda, as R' Yehuda deems a minor fit, and his statement here comes to elucidate and not to dispute.