Ivan Seeking said:
Many things in science do not have precise definitions. For example, biologists can't agree on a good definition for the word life. We all know what life is. We can almost always identify it and we also know when something is not life. Still, there are gray areas like viruses.
So, I don't expect to be able to define crazy to everyone's satisfaction. Still, we can characterize what a crazy claim is in the context of science.
Note that something "crazy" in science may not be crazy in other human frameworks. People talk of miracles in religion and this is outside the domain of science. If one believes that a God can suspend physical laws, then there is no point talking about using science to disprove or prove it. However, this is PF and we are talking about science here.
In the context of science, a crazy claim is NOT something like relativity and QM which is backed by experiments on top of experiments. If your Dad does not accept these ideas it's because he is uneducated about the experimental evidence. Many people reject the idea of natural selection as a viable explanation for evolution. Some even deny that evolution has taken place. etc. etc. These beliefs are not scientific viewpoints. This is not the type of thing we are talking about here.
These above examples are good to talk about because without experimental evidence, they would seem crazy to some people. Indeed, without previous theories failing to agree with experiments, there would have been no need to consider these newer theories. Anyone suggesting these ideas when existing theories perfectly match experiments, would be a little crazy. Note the difference, however. They and their ideas would be crazy, but at least it would be honest and not fraudulent craziness. If a crazy idea was proposed without any experimental or theoretical need, and it turned out to be right, then the inventor would be crazy and lucky. Still he would get the credit for his gamble, which is fair. Anyway these existing examples of "crazy ideas" were never crazy from a scientific viewpoint. They were developed to explain known experimental data and known failings in existing theories.
In science, a crazy claim or idea is one that is proposed in the absence of any need to explain credible experimental data AND in the absence of a failure of existing theory. The idea is particularly crazy when accepting the unneeded idea topples, or requires major revamping of, existing well-established science. In other words, you gain little and lose much in accepting the idea. It is even more crazy when it is based on the word of a human being just making a statement (e.g. i don't need to eat or drink). It is crazy to accept such claims when you have many very ordinary and plausible explanations (consistent with existing well-excepted ideas) that explain your questionable unverified data... I could go on trying to characterize, but one either gets the point, or one doesn't.
Just like I know a running dog is life, and a rock is not life, and a virus is a gray area, even without a clear definition of life; I know that QM is not crazy, and this "holy-man's claim" is crazy, and cold-fusion is a gray area, even without a clear definition of crazy.