Ad Hoc

An argument A is offered after the fact to explain an event X, in order to allege that A caused X. However no attempt is made to show that argument A is true or even related to X.

Examples

The lightning was bright and powerful, because the god Thor made it.

Everyone in the town died from the excruciating plague except one girl, because God favored her.

The attacks were perpetrated by foreign terrorists, because no one else could have.

Weaknesses

Ad Hoc is a bit of trickery, meant to make the speaker appear that he is offering conclusive logic when in truth the speaker is either being lazy or is unable or unwilling to prove argument A.

 * You can point out that the speaker has not proven A.
 * You can inquire as to whether he is merely being lazy.
 * If the speaker is evasive then you can pester the speaker.

Because Ad Hoc is merely an explanation and not a logical argument, its use by the speaker erodes his believability and perhaps, if the topic is weighty, his reputation. You can shame him into offering proof for argument A. Ask, surely that's not taboo to discuss? If the speaker says it is, demand to know why it is, when other topics are not off-limits.

If the speaker truly cannot justify A, you can expect anger, evasion, accusation, expulsion, praise for fantasy-based beliefs, outright retreat or even, if the speaker is a backward person, threats of violence.