The fallacy of illicit conversion (also known as false conversion) is usually easy to spot. But when used skillfully by someone who intends to deceive, this fallacy can trick even careful listeners.

The illicit conversion fallacy takes this form:

  • All P are Q.
  • Therefore, all Q are P.

So, if we were to say:

  • All Chinese speak the Chinese language.
  • Therefore, all Chinese language-speakers are Chinese.

We would be committing the logical fallacy of false conversion (or illicit conversion).

While it’s easy to spot the logical fallacy in the above example, it becomes slightly more difficult to spot the fallacy when we’re talking about negative statements.

The illicit conversion fallacy takes this form:

  • Some dog-owners are not participating in dog-fighting rings.
  • Therefore, some members of dog-fighting rings are not dog owners.

This is the sort of fallacy that can trip up unwary listeners if you don’t give them time to examine your (invalid) logic. Although the conclusion may be true, we haven’t proven it with this fallacious logic.

What adds to the confusion is that this sort of ‘conversion’ is not fallacious in all cases.

Consider the four types of statements you can make:

  1. Some are…
  2. None are… (or, ‘all are not’)
  3. All are…
  4. Some are not…

You can ‘convert’ statements 1 and 2 without committing a logical fallacy. The fallacy of illicit conversion only rears its ugly head in statements 3 and 4.

So, for case 1 (Some are…):

  • Some beagles are brown and white dogs.
  • Therefore, some brown and white dogs are beagles

And for case 2 (None are…):

  • No beagles are fast dogs.
  • Therefore, no fast dogs are beagles.

In both those examples, the logic is valid.

For case 3 (All are…)

  • All beagles are brown and white dogs.
  • Therefore, all brown and white dogs are beagles

For case 4 (Some are not…)

  • Some beagles are not good at hunting rabbits.
  • Therefore, some dogs who are not good rabbit hunters are not beagles.

In both those examples, the logic is invalid because of the fallacy of illicit conversion.

This fallacy gets more tricky when you (either intentionally, or not) omit the “some” or “all”, and especially when you dress it up with lots of unneeded verbiage:

  • Decades of research by the FBI and other international crime-fighting organizations shows that serial killers start out killing animals at a young age. Over time, their crimes progress in severity until they’re engaged in a repetitive cycle of murderous violence.
  • Therefore, it is important for us to be proactive in detecting and intervening in situations where it’s determined that youngsters have killed animals, because without immediate corrective actions by parents, community members, and law enforcement authorities, these youngsters will grow up to be the worst of the worst of antisocial offenders.

Actually no, that conclusion isn’t valid because of the fallacy of false conversion.

The above example becomes logically valid by adding the word “some” in two places:

  • Some serial killers start out by killing animals.
  • Therefore, some children who kill animals will turn into serial killers.

Illicit Conversion Joke

A guy walks into a bar and says to the bartender, “All lawyers are creeps!” From down at the end of the bar, a drinker complains, “Hey! I resent that!” The man asks, “Why? Are you a lawyer?” The drinker replies, “No, I’m a creep!”

OK, lawyer jokes are always funny, but there is nothing funny about the illicit conversion in that joke! Just because all lawyers are creeps doesn’t mean all creeps are lawyers.