Mishnah 4 in the third chapter of Maseches Kiddushin presents a famous dispute about the very definition of a condition: what makes something a valid condition, and what requirements must be met for it to take effect.
Rabbi Meir says: "Kol tenai she'eino k'tenai bnei Gad uvnei Reuven, eino tenai" - any condition that a person attaches to an action he is about to perform, if it is not structured like the condition made by the children of Gad and the children of Reuven with Moshe Rabbeinu, is not a valid condition at all.
The episode itself went like this: the children of Gad and the children of Reuven asked to receive their portion on the eastern side of the Jordan, instead of the usual portion in Eretz Yisrael on the western side of the river. They told Moshe Rabbeinu that if they would receive their portion on the eastern side, they would cross the Jordan and help conquer the rest of the land. An agreement was therefore struck between them and Moshe Rabbeinu: they would receive their portion on the eastern side of the Jordan, on condition that they first go out to war.
Three requirements are derived from the condition of the children of Gad and the children of Reuven:
A double condition - the condition was stated both in the positive and in the negative: if they cross the Jordan and fight, they will receive the portion; and if not, they will not receive it. According to Rabbi Meir, for a condition to take effect it must be stated in both ways: if the matter is done, the act will take effect; and if it is not done, it will not take effect.
The condition precedes the act - the statement of the condition must come before the statement of the act: "If we cross the Jordan, we will receive a portion on the eastern side," and not the reverse - "We will receive a portion on the eastern side if we cross the Jordan." That formulation is not a condition preceding the act but an act preceding the condition, and that is not the proper order.
The positive precedes the negative - one must open with the positive side, as the children of Gad and the children of Reuven did: "If we cross and fight, we will receive our portion; and if we do not cross, we will not receive it" - and not in the reverse order, saying first "If we do not fight, we will not receive a portion," and only afterward "and if we do fight, we will receive."
The source for all this is in the verse: "Vayomer Moshe aleihem: im ya'avru bnei Gad uvnei Reuven" - that is, that they would cross the Jordan armed and fight, and it is written: "Ve'im lo ya'avru" - in which case they would not receive that portion. The Mishnah focuses on the first requirement, the need for a double condition, stating the condition in both directions, but alongside it the other two components are required as well.
The view of Rabbi Chananya ben Gamliel:
Rabbi Chananya ben Gamliel disagrees, at least with the requirement of a double condition itself, and says: "Tzarich hayah hadavar lomro". In his view, no proof can be brought from the children of Gad and the children of Reuven, because the need to state the condition in both directions did not stem from the fact that without it the condition would be void, but from a practical reason: had the negative side not been stated - that if they do not cross the Jordan they will not receive this portion - one might have understood that they would not inherit in the land of Canaan either, meaning that if they failed to fulfill the condition they would not receive a portion even on the western side of the Jordan, and that is not the case.
Therefore it was necessary to spell out the negative side, that if they do not cross the Jordan they will receive their portion in the land of Canaan like all the other tribes. Had this not been stated, there would have been room to err and think that by failing to fulfill the condition they would forfeit even their ordinary portion. It emerges that there is no proof from here that every condition must be doubled and formulated in both directions.
As we mentioned, opinions differ as to whether Rabbi Chananya disagrees with the other two requirements as well - the positive preceding the negative and the condition preceding the act - and these two points are also open to question.
The halachah:
The halachah follows Rabbi Meir, and all of these requirements apply when the condition is introduced with the word "if" - "if I do such and such." However, when one uses the expression "al menas" - "on condition that..." - none of these requirements is necessary: not a double condition, not the order of the statement, and not any of the other details, and the condition is effective in any case.
In summary: in this Mishnah we learned the three requirements that Rabbi Meir derived from the condition of the children of Gad and the children of Reuven - a double condition, the condition preceding the act, and the positive preceding the negative; the view of Rabbi Chananya ben Gamliel, that no proof can be brought from there because the verse needed to spell out the negative side for a different reason; and the halachah, which follows Rabbi Meir when the word "if" is used, whereas with the expression "al menas" the condition is effective even without any of these details.