Every test of a theory, whether resulting in its corroboration or falsification, must stop at some basic statement or other which we decide to accept. If we do not come to any decision, and do not accept some basic statement or other, then the test will have led nowhere. But considered from a logical point of view, the situation is never such that it compels us to stop at this particular basic statement rather than at that, or else give up the test altogether. For any basic statement can again in its turn be subjected to tests, using as a touchstone any of the basic statements which can be deduced from it with the help of some theory, either the one under test, or another. This procedure has no natural end.

It is a test of true theories not only to account for but to predict phenomena.

(…) Theories are neither right nor wrong. They are attempts at explaining the inexplicable. It is not the theory that matters, but the way it is being tested. (…) Experiment with any theory you like – if you are truly earnest and honest, the attainment of reality will be yours. (…) It is the earnestness that liberates and not the theory.

For the sophisticated falsificationist a scientific theory T is falsified if and only if another theory T' has been proposed with the following characteristics :

Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory. As philosopher of science Karl Popper has emphasized, a good theory is characterized by the fact that it makes a number of predictions that could in principle be disproved or falsified by observation. Each time new experiments are observed to agree with the predictions the theory survives, and our confidence in it is increased; but if ever a new observation is found to disagree, we have to abandon or modify the theory.

You have no proof of this," I said.
"No. But a theory must be tested to be disproved, Mr Bastable.

Falsifiability Least understood, but the most important principle for anyone claiming “science” on their side — falsifiability. If it doesn’t make falsifiable predictions, it’s not science. For you to believe something is true, it should have predictive power, and it must be falsifiable. [11]

One truth we have been able to count on concerning scientific persuit over the centuries has been that only testable theories survive the intense scrutiny of experimental science.

«A theory is part of empirical science if and only if it conflicts with possible experiences and is therefore in principle falsifiable by experience.»

The two psycho-analytic theories were in a different class. They were simply non testable, irrefutable. There was no conceivable human behav­iour which could contradict them. This does not mean that Freud and Adler were not seeing certain things correctly: I personally do not doubt that much of what they say is of considerable importance, and may well play its part one day in a psychological science which is testable. But it does mean that those ‘clinical observations’ which analysts naively believe confirm their theory cannot do this any more than the daily confirmations which astrologers find in their practice. And as for Freud’s epic of the Ego, the Super-ego, and the Id, no substantially stronger claim to scientific status can be made for it than for Homer’s collected stories from Olympus. These theories describe some facts, but in the manner of myths. They contain most interesting psychological suggestions, but not in a testable form.

A theory is only as good as its assumptions. If the premises are false, the theory has no real scientific value. The only scientific criterion for judging the validity of a scientific theory is a confrontation with the data of experience.

Knowledge is power. The real test of knowledge is not whether it is true but whether it empowers us. Scientists usually assume that no theory is 100% correct. Truth, consequently, is a poor test for knowledge. The real test is utility. A theory that enables us to do new things constitutes knowledge.

Good tests kill flawed theories; we remain alive to guess again.

A theory which is not refutable by any conceivable event is nonscientific. Irrefutability is not a virtue of a theory (as people often think) but a vice.