Niddah, Chapter 5, Mishnah 1. The Mishnah opens the new chapter with an unusual case of childbirth and asks whether it carries the halachic consequences of an ordinary birth at all.
By way of background: a woman who gives birth enters a period of yemei tum'ah, days in which she is temei'ah on account of the birth, and afterwards a period of yemei taharah, days in which blood that she sees does not render her temei'ah. When those periods are over she brings her korban leidah. The question of our Mishnah is whether all of this applies to a birth that did not take place in the normal way.
Yotzei Dofen
The case is "yotzei dofen", literally a child that emerged through the wall of the body, that is, a delivery by Caesarean section rather than through the ordinary passage. Is such a mother a yoledes in halachah?
The first Tanna states his position in two parts. First, "ein yoshvin alav yemei tum'ah viyemei taharah": no days of tum'ah and no days of taharah are counted on account of such a delivery. And then he adds, "ve'ein chayavin alav korban": there is likewise no obligation to bring an offering for it. Nothing that would normally be set in motion by a birth happens here, neither the tum'ah that a birth brings, nor the stretch of pure days that follows it, nor the korban at its conclusion. For the Tanna Kamma, a woman who delivers in this way simply does not fall into the category of a yoledes.
The Source of the Ruling
According to this opinion the ruling is a gezeiras hakasuv, a decree derived from the wording of the pasuk. The Torah writes "ishah ki sazria veyaldah", a woman who conceives and gives birth. The Torah ties the birth to the same route by which conception took place, meaning that the child must emerge through the ordinary passage. Where the child did not come out that way, the verse's decree excludes her from the halachos of a yoledes.
Rabbi Shimon's View
Rabbi Shimon disagrees: "harei zeh keyalud", this child is regarded as one who was born, and the mother has the full standing of a yoledes. She has yemei tum'ah, she has yemei taharah, and she is obligated in the korban, exactly as though she had delivered in the conventional way.
So our Mishnah leaves us with a machlokes Tannaim over a single question: is childbirth defined by the fact that a child came into the world, as Rabbi Shimon holds, or by the manner in which the Torah describes a birth taking place, as the Tanna Kamma holds.