Ad Hoc hypothesis

Made up for the specific purpose, case, or situation at hand and for no other.

19th February 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 pertinent), 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 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.


  • 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.


  • 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!