In argumentation, an 'ad hoc hypothesis' is usually introduced for the purpose of maintaining a person's belief or position by explaining away anything that's contradictory. They are normally introduced in response to individual cases or arguments instead of being universally applicable. Ad-hoc responses aren't necessarily fallacious (some could be pertinent), but because of their purpose, they often are.
Their basic structure is:
- X is claimed to be true;
- Y is presented to show that X isn't true;
- Y is dismissed with Z (the ad-hoc excuse) to maintain the belief that X is true.
As ad-hoc hypotheses are usually made up on the fly, their quality is usually very low. The bottom line with ad-hoc responses however, is that simply making up potential reasons why a challenging counter-claim could be dubious or false doesn't actually show that it is the case. The burden is on the person making the ad-hoc case to back it up.
In longer debates, a series of ad-hocs can occur. Their presence is a tell-tale sign of someone defending a non-evidence-based belief, especially when the ad-hoc excuses contradict each other.
Examples:
- Dowsing works, I'm close to 100% accurate!
- The results of your double-blind test show that you scored at the chance level of 10%.
- Ah, but there is water in an underground stream that interfered with the test.
and
- Mediums give fantastic demonstrations of afterlife communication night after night in theatres up and down the country.
- Why can't they provide this evidence under test conditions?
- They can't just switch it on and off you know. They're not performing monkeys!