Pathagorous showed that if he assumed that the square root of 2 could be expressed as a fraction of whole numbers, then the numerator and denominator of this fraction would each have a factor of 2 which would violate the law that this fraction could be expressed in lowest terms to begin with. Thus he concluded that their are some numbers that cannot be expressed as a fraction of whole numbers. Or as another example, let's suppose that it is possible to travel back in time. If this were possible, then you could go back in time to before you went back in time and prevent yourself from going back in time. Since your going back in time could prevent your going back in time, it is not possible to go back in time.
Lawyers try to get their opponent to change their minds.
For if even the opposition proves his case, then the lawyer has won the case and their can be no more argument. Physical systems express this principle as stable equilibrium. Stable equilibrium occurs when a force compels an object into a certain state - such as a ball inside a bowl. The fact that the ball is not at the bottom of the bowl means that the ball will role towards the bottom of the bowl. Therefore, we would expect the ball to come to rest at the bottom of the bowl.