The ad-hoc argument is my favourite underhanded debate technique. Why? Because people use it all the time, and most of them don’t even know it exists. Everyone knows ad-hominem, post-hoc and the straw-man – well, that number shrinks if you exclude all the people who misuse the term straw-man – but almost nobody knows the ad-hoc argument.
The ad-hoc argument is a technique used to explain away a lack of evidence for your pet theory. James Randi, a well-known skeptic, offers a one million dollar cash reward to anyone who can demonstrate psychic ability under controlled laboratory conditions. One of his tests involves the psychic guessing playing cards that Randi deals from a deck in another room. “What card am I holding?”
So far, no one has won the million dollars.
But some alleged psychics blame their failure on Randi’s negative vibes. Randi’s hostile attitude is messing with their ability. This is the ad-hoc argument. We tested a claim, we found the evidence for it lacking and now someone needs to explain away results they don’t like.
One key component of the ad-hoc argument is the fact that the explanation is something we cannot test. This isn’t an attempt to control the variables of an experiment. I mentioned a few weeks ago that a drug trial might be considered invalid if the researchers find out that the trial participants used other medications before conducting the experiment. That’s something we can check.
There is no way to determine whether or not Randi has a hostile attitude, much less whether or not a hostile attitude would interfere with psychic ability. To determine whether a hostile attitude would interfere with psychic ability, we must first establish the existence of psychic ability.
Another common example of this fallacy looks like this.
Birther: “Obama was born in Kenya!”
Skeptic: “Then why does his birth certificate say otherwise?”
Birther: “Well, clearly that was a forgery that he made after the fact.”
The ad-hoc argument is the last resort of people who just don’t want to accept that the facts just don’t support their position.
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