Niddah, Chapter 6, Mishnah 2. Our Mishnah deals with the physical signs by which a girl is recognized as having left childhood behind and become a gedolah, an adult. Two such signs are spoken of here. The siman hatachton, the lower sign, is the growth of two hairs. The siman ha'elyon, the upper sign, is the development of the breasts. The question the Mishnah takes up is what the halachah says when one of these signs is present and the other is not, and the practical setting in which the question is tested is chalitzah and yibum: a widow who has no children from her husband either marries his brother (yibum) or is released by him through the ceremony of chalitzah, and only an adult woman is fit to perform either of these acts.
Consider the first situation the Mishnah presents. Its words are "ba siman hatachton", the lower sign has arrived, "ad shelo ba ha'elyon", while the upper one has not yet appeared. Nothing here presents a difficulty. The two hairs have grown, and with their appearance the girl has entered adulthood, so if she is a widow standing before chalitzah or yibum she is fit for either path: "o choletzes o misyabemes."
The second situation is where the discussion lies. Here the Mishnah describes "ba ha'elyon", the upper sign has come, while "ad shelo ba hatachton", the lower sign has yet to appear. To this the Mishnah attaches a comment of its own, "af al pi she'ee efshar": a case like this cannot genuinely occur. The Gemara explains that nature does not operate this way, and the breasts never develop ahead of the growth of the two hairs.
What, then, do we do with the girl in front of us who appears to be in exactly that state? Rebbi Meir rules "lo choletzes velo misyabemes" - she performs neither chalitzah nor yibum. Since what we seem to be seeing cannot really be, we have no reliable evidence of adulthood before us, and she is treated as one who has not yet become a gedolah.
The Chachamim disagree: "o choletzes o misyabemes" - she is an adult, and she may perform chalitzah or yibum. Their reasoning follows from the very same premise. Precisely because the upper sign cannot arrive on its own, the moment we see it we know that the two hairs of the lower sign did in fact grow; they simply fell out afterwards and are no longer visible to us. The absence of the lower sign is therefore not the absence of adulthood, only the absence of something we can currently observe.
Finally the Mishnah lays bare the rule that stands behind both positions, opening with "mipnei she'amru", for they have said. Regarding the lower sign, in its words "efshar latachton lavo", it can indeed come "ad shelo ba ha'elyon", in advance of the upper one, "aval ee efshar la'elyon lavo", whereas the upper sign has no way of arriving "ad shelo ba hatachton", ahead of the lower one. Rebbi Meir and the Chachamim both accept this rule; their dispute concerns only the inference to be drawn from it. Rebbi Meir holds that a presentation nature does not allow leaves us nothing to rely upon, so adulthood remains unproven. The Chachamim hold the reverse: the very impossibility carries the proof, for if the upper sign stands before us, the lower sign must have come and gone.