Kilayim, Chapter 5, Mishnah 2. This mishnah deals with a vineyard whose vines are planted unusually close to one another, and with the question of whether such a planting is considered a vineyard in every respect.
The text of the mishnah and its subject:
"Kerem shehu natua al pachos me'arba amos" - a vineyard whose vines are planted less than four amos apart. This distance is too close for the vines to grow properly, since they require a space of at least four amos.
"Rabbi Shimon omer: eino kerem" - since the vines are too close together and cannot grow properly, this planting does not have the status of a vineyard.
"VaChachamim omrim: kerem, v'ro'in es ha'emtza'iyos k'ilu einan" - the Sages disagree and hold that it does have the status of a vineyard. The middle vines, those planted unusually close, are regarded as though they do not exist; and once we disregard them, the vines beyond them stand at the proper distance from one another - more than four amos, six amos, or whatever the measurement happens to be - and we have a valid vineyard.
The limit of this ruling - only two rows:
Even according to the Sages, if we have before us only two rows planted unusually close, we cannot apply "we regard the middle ones as though they do not exist." For once we remove one row, even theoretically, a single row remains - and that is not a vineyard. Therefore the Sages would agree that with only two rows this ruling does not help.
The reasoning behind the dispute according to the Gemara in Bava Basra:
According to Rabbi Shimon: a person does not plant his vines so close together, and he certainly does not plant in a manner in which he knows in advance that he will have to uproot. For this reason it is clear that this is not a vineyard, since a person does not do such a thing; and if it happened by mistake or for some other reason, it was done improperly and is of no effect.
According to the Sages: sometimes a person plants in a way that appears improper, because he is testing which plantings will take root and which will not. This is not his long term plan, and he expects that he will have to remove some of them at a certain stage. Therefore, even though he has not yet removed them, since their removal is part of his plan, those plantings destined to be uprooted can be regarded as though they do not exist already now - and for this reason it has the status of a vineyard even from now.
In summary: in this mishnah Rabbi Shimon and the Sages disagree about a vineyard planted with less than four amos between the vines. According to Rabbi Shimon it is not a vineyard, since a person does not plant this way to begin with; and according to the Sages it is a vineyard, since "we regard the middle ones as though they do not exist" - the closely planted vines are destined to be uprooted and are considered as though they do not exist, provided that more than two rows remain.