I already touched on this fallacy in my post about the Appeal to Ignorance, but I wanted to cover it in more depth. The way to spot a general use of the Special Pleading fallacy is to notice when someone attempts to cite something as an exception to a generally excepted principal or rule without justifying the exception. What does it look like?
Person A: Homeopathy uses spiritual imprints of toxins to make people develop a response to them.
Person B: Several laws of physics would have to be violated if homeopathy worked like that.
Person A: It works, but it does so in a way that can’t be tested or known by science.
So why does homeopathy (or your claim of choice) get to ignore scientific testing and still be valid? That is never convincingly argued. Science is not some narrow way of looking at things that can only examine part of the universe. Every single phenomena in the universe falls under the purview of science, in principal. Even if we’re talking about a supposed supernatural phenomena, like prayer or homeopathy, science can still test and falsify the claim. We can test whether or not praying for sick people helps them or praying over plots of land makes plants grow better. As it turns out, praying doesn’t do anything to make things happen in a reliable way.
Special Pleadings come up a lot in religious debates as well. Whenever anyone says, “The Lord works in mysterious ways” they are employing a Special Pleading. This is because the “mysterious” thing they’re talking about (usually used to try and rebut the problem of evil) can’t be explained in a way that makes any sense or is convincing. Therefore, they say that their claim is immune from the normal standards of reason and evidence that we use for everything else.
It is highly possible that there are phenomena that are real and yet understood by science. The dawn of radio technology is a historical example. However, the only rational thing to say if someone shows you a radio transmission for the first time is, “I wonder how that works.” It is not rational to leap to an explanation right away, or endorse a ludicrous one if there are better ones. One way that radio signals might work is invisible fairies carrying little bits of information and flying from one radio to another. If someone told you that, and you didn’t know how radios really worked, would it be irrational to not accept this explanation? No. Not if the existence of fairies wasn’t already established. If fairies were a known species (phenomenon?) then the idea that they might be also be responsible for radio signals would be plausible. However, you can’t invoke a new, outlandish claim to explain an unknown phenomena. If someone said Leprechauns or gods were responsible for the faster-than-light neutrinos, we aren’t required to take them seriously. Bottom line, you can’t bring in a new idea to explain an unusual phenomena unless there is already independent evidence for that explanation or there are some really compelling reasons.
Why is it not scientific or rational to say something like “God did it?” For one, you need independent evidence for you explanation and a proposed mechanism. For another, you can’t make new predictions or anticipate possible states of the world given that explanation. If God is responsible for rainbows, what other phenomena can we anticipate? If light refraction is responsible for rainbows, we can expect to be able to reproduce rainbows at will, like in sprinklers.
Bottom line: Be on the lookout for anyone who says their claim is immune from the normal standards of evidence and testing that apply to everything else. If they can’t give you a satisfactory explanation for why their claim is above the rules, then it isn’t.