Rabbi Meir says: Any condition that is not like the condition of the children of Gad and the children of Reuven is not a [valid] condition; and that is what is written “If you do well... and if you do not do well.”
On the daf / in context: The doubled positive-and-negative formulation is cited as the model of a valid halachic condition.
Type of Learning: Condition כפול / תנאי בני גד ובני ראובןWhat Is Learned: A valid condition is stated in both its positive and negative forms, following the model of the בני גד ובני ראובן.