Relativist fallacy

O'Reilly: I'm sticking with Judeo-Christian philosophy and my religion of Roman Catholicism, because it helps me as a person. Dawkins: Ah, that's different. If it helps you, that's great. That doesn't mean it's true. O'Reilly: Well, it's true for me. You see, I believe — Dawkins: You mean true for you is different from true for anybody else? The relativist fallacy (or subjectivist fallacy) is a logical fallacy that occurs when it is asserted that some fact may be true for some people but not true for others.

Epistemic relativism, moral relativism, or cultural relativism are not necessarily relativistic fallacies. Relativistic fallacies only occur when someone uses a mid-argument, ad hoc switch to a relativistic view of truth in order to defend some position, which they had previously asserted was objective. It is commonly used to defend beliefs that are based on subjective experiences (such as the efficacy of prayer or homeopathy).

The fallacy is an informal fallacy.

Form
At heart, the relativist fallacy adopts a purely subjective view of truth, in that only what someone thinks is true can be said to be true, and it is impossible to prove otherwise (i.e., is unfalsifiable). This leads to the syllogism:

Or in conversation:

Explanation
If someone said that "the world is round", and someone else said that "that might be true for you, but not for me", then the second person would appear to be living on a different world than the rest of the human race (perhaps literally). The second person appears to be ignoring all evidence for a round world, and thus holding a fallacious position.

More commonly, this tactic is used to defend beliefs when someone realizes that there is a critical problem with their position, but doesn't want to recognize it.

Demarcation
When people say "X is true for me", they often truly mean "X is true about me". For example: if Ann, a 4-foot-tall dwarf miner, weighs 200 pounds, she is overweight. If Bob, an 8-foot-tall ogre lord, weighs the same, then he is underweight. Because these facts are subjective, it is not fallacious to consider them as such.