If a theory claims to be able to explain some phenomenon, but does not generate even an attempt at an explanation, then it should be banished.

A theory should not attempt to explain all the facts, because some of the facts are wrong.

On your way towards becoming a bad theoretician, take your own immature theory, stop checking it for mistakes, don't listen to colleagues who do spot weaknesses, and start admiring your own infallible intelligence.

If the fact will not fit the theory - let the theory go

If two opposite theories are propagated one will be wrong.

A very interesting theory makes no sense at all.

In the end, a theory that can’t be shown to be wrong can never be shown to be right.

If you can’t think of an observation that could disprove a theory, that theory simply isn’t scientific.

The derivation of a judgment on the Urheimat question from the alleged motives of the proponents of the contending theories is all-pervading and vitiates the whole debate. Yet, if a theory can be considered wrong simply because it is being used for political ends, it is clear that the AIT itself must be the wrongest theory in the world: one looks in vain for a historical hypothesis which has been more tainted with various political uses including the most lethal ones.

I call it theory-induced blindness: once you have accepted a theory and used it as a tool in your thinking, it is extraordinarily difficult to notice its flaws. If you come upon an observation that does not seem to fit the model, you assume that there must be a perfectly good explanation that you are somehow missing.

If the evidence says you’re wrong, you don’t have the right theory. You change the theory, not the evidence.

When the fact doesn't meet the theory then let go the theory.

(…) 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.

A theory that explains everything, explains nothing