Illicit Conversion — The Trick You Don't See Coming
Also known as: Converse Error of Categorical Propositions, Simple Conversion Error
🔥 Hook
"All terrorists are extremists.
Sound familiar? This happens more than you think.
🧠 What's Actually Happening?
Illicit conversion is a formal fallacy that involves invalidly converting a categorical statement by switching its subject and predicate. While 'No A are B' validly converts to 'No B are A,' and 'Some A are B' converts to 'Some B are A,' the statement 'All A are B' does not validly convert to 'All B are A.' This asymmetry is frequently overlooked in everyday reasoning.
Here's the sneaky part: Symmetric phrasing feels natural -- if A relates to B, B should relate to A in the same way. This intuition is correct for some logical forms but incorrect for universal affirmatives.
📱 Real-Life Scroll
What you'd see online:
"All terrorists are extremists. Therefore, all extremists are terrorists." (Being a terrorist implies being an extremist, but being an extremist does not imply being a terrorist.)
Another one
'All vaccines are medical interventions' is incorrectly converted to 'All medical interventions are vaccines.' Being a vaccine guarantees being a medical intervention, but surgeries, antibiotics, and therapies are also medical interventions without being vaccines.
What it looks like IRL:
Pervasive in security profiling ('all terrorists are X, therefore all X are potential terrorists'), medical reasoning, and everyday categorical judgments that conflate subset with identity.
🔍 How to Spot It
Test the conversion with a clear counterexample. Visualize with sets: all terrorists fall within extremists, but extremists is a larger set containing many non-terrorists.
Quick checklist:
- ✓ Is the argument actually proving what it claims?
- ✓ Could I explain this to a friend without it falling apart?
- ✓ If I remove the emotion/pressure, does it still make sense?
💬 What You Can Do
When someone hits you with this, try: "Interesting, but does that actually follow?" You don't need to win. You just need to not get fooled.
🎯 Your Challenge
Find one example of illicit conversion this week. Could be anywhere — a debate, a comment section, a news article, or even your own reasoning. Write it down. The moment you can name it, it loses its power.
Part of the TellDear Teen Book — criticalthinking.guide