Ad Hoc hypothesis fallacy

Ad hoc hypothesis: made up for the specific purpose, case, or situation at hand and for no other.

by John Jackson © 2013

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 valid), but because of their purpose, they often are.

Their basic structure is:

  1. X is claimed to be true;
  2. Y is presented to show that X isn't true;
  3. Y is dismissed with Z (the ad-hoc excuse) to maintain that X is true.

As ad-hoc hypotheses are usually made up on the fly, their quality is usually very low - lacking in sound reasoning or evidence. The bottom line with ad-hoc responses however, is that simply making up potential reasons why a 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!