Affirming a Disjunct — The Trick You Don't See Coming
Also known as: Affirming One Disjunct, Fallacy of the Alternative Disjunct
🔥 Hook
"She's either at the library or at the coffee shop.
Sound familiar? This happens more than you think.
🧠 What's Actually Happening?
Affirming a disjunct is a formal fallacy that occurs with inclusive disjunctions (OR statements). Given 'A or B' and knowing A is true, the fallacy concludes that B must be false. This is invalid because 'or' in logic is inclusive by default -- both A and B can be true simultaneously. The error lies in treating an inclusive 'or' as exclusive.
Here's the sneaky part: In everyday language, 'or' is often used exclusively ('soup or salad'), conditioning people to assume that confirming one option eliminates the other.
📱 Real-Life Scroll
What you'd see online:
"She's either at the library or at the coffee shop. I just confirmed she's at the library. Therefore, she's definitely not at the coffee shop." (She could have been at both at different times, or the statement could allow both.)
Another one
The ad says 'Buy our juice for vitamins or great taste!' A customer thinks: 'I bought it for the vitamins, so it definitely can't also taste great.' But both could be true simultaneously — the disjunction doesn't exclude the other option just because one is confirmed.
What it looks like IRL:
Appears in everyday reasoning about alternatives, legal interpretation of statutes using 'or,' and diagnostic reasoning where confirming one possibility prematurely eliminates others.
🔍 How to Spot It
Clarify whether the disjunction is inclusive or exclusive. In logic, 'A or B' allows both to be true unless explicitly stated as exclusive ('either A or B, but not both').
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 affirming a disjunct 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