Gittin, Chapter Two, Mishnah 6. This mishnah deals with people who are not qualified to bring a get and serve as an agent, and with special cases in which their status changed in the middle - between the time they received the get and the time they delivered it.
The invalid cases - one who was not of sound mind at the time he received the get:
"Kibel hakatan v'higdil" - a minor who received the get in order to serve as an agent, and before he delivered it he reached the age of mitzvos.
"Cheresh v'nispakeach" - he was deaf at the time he received the get, and afterwards his hearing was restored.
"Suma v'nispakeach" - he was blind at the time he received the get, and afterwards he became able to see.
"Shoteh v'nishtapeh" - he was mentally incompetent, and afterwards he was healed and became of sound mind.
"Nochri v'nisgayer" - he was a gentile at the time he received the get, and afterwards he converted.
In all of these cases the ruling is "pasul" - they are not qualified to bring the get, since at the time they received the get they were not qualified to do so.
The valid cases - one who was of sound mind both at the beginning and at the end:
"Pikeach v'niscaresh v'chazar v'nispakeach" - he could hear, then became deaf, and afterwards his hearing returned to him.
"Pasuach v'nistama v'chazar v'nispatach" - he could see, then became blind, and afterwards his sight returned.
"Shafui v'nishtateh v'chazar v'nishtapeh" - he was of sound mind, then became mentally incompetent, and afterwards returned to his sound mind.
In all of these the ruling is that they are qualified. And this is the principle the mishnah establishes: "Kol shetechilaso v'sofo b'da'as kasher" - as long as they possessed full awareness at the beginning and at the end, it is valid, even though in the middle they lost that awareness and that ability. For at the time they received the get they were qualified, and at the time they gave the get they were qualified, and what happened in between makes no difference.
A precise reading of the words "b'da'as":
The very use of the expression "kol shetechilaso v'sofo b'da'as" - a matter of awareness specifically - excludes the blind man, since in his case nothing depends on awareness at all, but only on the ability to see. In his case, if he was able to see at the time he received the get, he can say "b'fanai nichtav u'v'fanai nechtam" (it was written in my presence and signed in my presence), and he does not need his sight to return to him. Even if he cannot see at the time he delivers the get, that is of no consequence; it is enough that he could see at the beginning, when he originally received the get.
This is why the principle is phrased specifically as "its beginning and its end with awareness," and not in terms of ability or of qualification to serve as an agent. For in the case of the blind man there is no need for his sight to return, and as long as he had sight at the beginning, the get is valid.
In summary: in this mishnah we learned that what determines the matter is the agent's status at the time he receives the get and at the time he delivers it: one who at the time of receiving was a minor, deaf, blind, mentally incompetent or a gentile is invalid, even though his status changed afterwards; whereas one who was of sound mind at the beginning and at the end is valid, even though he was disqualified in the middle. And the principle of it all: "kol shetechilaso v'sofo b'da'as kasher," and in the case of a blind man, sight at the beginning alone is enough.