"How is it possible that we can extend our laws into regions we are not sure about? Why are we so confident that, because we have checked the energy conservation here, when we get a new phenomenon we can say it has to satisfy the law of conservation of energy? Every once in a while you read in the paper that physicists have discovered that one of their favorite laws is wrong. Is it then a mistake to say that a law is true in a region where you have not yet looked? If you will never say that a law is true in a region where you have not already looked you do not know anything. If the only laws that you find are those which you have just finished observing then you can never go on and try to make guesses. So what we always do is to stick our necks out, and in the case of energy the most likely thing is that it is conserved in other places.
Of course this means that science is uncertain; the moment that you make a proposition about a region of experience that you have not directly seen then you must be uncertain. But we always must make statements about the regions that we have not seen, or the whole business is no use. For instance, the mass of an object changes when it moves, because of the conservation of energy. Because of the relation of mass and energy the energy associated with the motion appears as an extra mass, so things get heavier when they move. Newton believed that this was not the case, and that the masses stayed constant. When it was discovered that the Newtonian idea was false everyone kept saying what a terrible thing it was that physicists had found out that they were wrong. Why did they think they were right? The effect is very small, and only shows when you get near the speed of light. If you spin a top it weighs the same as if you do not spin it, to within a very very fine fraction. Should they then have said, "If you do not move any faster than so-and-so, then the mass does not change"? That would then be certain. No, because if the experiment happened to have been done only with tops of wood, copper, and steel, they would have had to say, "Tops made of wood, copper, and steel, when not moving any faster than so and so..." You see, we do not know all the conditions that we need for an experiment. It is not known whether a radioactive top would have a mass that is conserved. So we have to make guesses in order to give any utility at all to science. In order to avoid simply describing experiments that have been done, we have to propose laws beyond their observed range. There is nothing wrong with that, despite the fact that it makes science uncertain. If you thought before that science was certain- well, that is just an error on your part.

-Richard Feynman