"Ad hoc assumptions"

An ad hoc assumption is an assumption introduced specifically to protect an argument from an apparent problem, rather than because there is independent evidence for it.

Example:

Theory:

"All rulers abolish slavery because they are humanitarian."

Evidence appears showing a ruler abolished slavery for military reasons.

Instead of revising the theory, someone says:

"Yes, but that ruler was humanitarian in addition to having military motives."

Then another contrary case appears:

"Yes, but that ruler had a secret humanitarian motive."

And another:

"Yes, but this ruler's humanitarianism was hidden."

If every counterexample produces another special explanation, the theory becomes increasingly ad hoc.

The problem is not that an additional explanation is automatically false.

The problem is that it is being introduced merely to rescue the theory, without independent support.