Karl Popper: The Importance of Falsifiability in Science

"I like to distinguish between the scientific method, which is applied to all materialistic theories. The scientific method is one of observation or experimentation, repeatability, falsification, the repeatability by different techniques of different labs, or prediction and results. It's a scientific method. I say that if it's a non-physical theory, or the theory has any aspect of non-physicalism to it, it doesn't have to be all, any aspect invalidates, to some degree invalidates, the scientific method as I just described from being applicable. And I think there's a tendency in some theories that have some non-physical aspect to try to show their legitimacy by applying the scientific method... I think that's a false desire, because once you're non-physical you are invalidating, to one degree or another, the scientific method. Maybe not totally, but in some degree." Robert Lawrence Kuhn

"No single theory ever agrees with all the facts in its domain" Paul Feyerabend