Kiddushin, Chapter Three, Mishnah Four
This mishnah contains a general principle of Rabbi Meir: any stipulation must be a double stipulation. This means that if I make a stipulation I must state both the consequences of the condition being fulfilled and the consequences of its not being fulfilled. For instance, if I want to say that I will come to your house if you give me chocolate cake (and I would), I must say, I will come to your house if you give me chocolate cake, and I will not come to your house if you dont give me chocolate cake. Otherwise the stipulation is not legally binding, and even if you give me chocolate cake, I am not legally bound to come to your house (but I would never do such a thing).
Rabbi Meir derives this principle from Mosess words to the children of Gad and Reuben, as we shall explain below.
Rabbi Meir says: every stipulation which is not like that of the children of Gad and the children of Reuben is not a [valid] stipulation, as it say, And Moses said to them, If the children of Gad and the children of Reuben will pass [with you over the Jordan, then you shall give them the land of Gilead for a possession]. and it is also written, But if they will not pass over with you armed, [then they shall have possessions among you in the land of Canaan] (Numbers 32:29-30).
Rabbi Hanina ben Gamaliel says: the matter had to be stated, for had it not been stated it would have implied that even in Canaan they should not inherit.
The children of Gad and Reuven did not want to inherit on the west side of the Jordan river; they wished to remain on the east side, in a place good for their cattle. Moses responded to them that if they cross to help fight in the conquering of Canaan, then they may inherit on the east side of the Jordan. He also added that if they did not cross and fight with the rest of Israel, then they would only be able to inherit in Canaan. From the fact that Moses doubled his stipulation, Rabbi Meir derives that all stipulations must be doubled.
Rabbi Hanina ben Gamaliel responds that Moses had to state the second half of his statement. Had he not done so, he might have implied that if they didnt fight for Canaan they wouldnt even get an inheritance in Canaan. Since the second half is a necessary statement, we cannot learn that stipulations that dont need a negative side do not need to be doubled.
The mishnah ends with Rabbi Hanina ben Gamaliels response to Rabbi Meir. Had Rabbi Meir responded he might have said that it is obvious that the children of Gad and Reuven would inherit in the land of Canaan, since all of the tribes inherited there irregardless of their participation in the conquest. Since the second half of his stipulation was unnecessary, we can learn that the only reason Moses added it in was because all stipulations must be thusly doubled.