Affirming a disjunct

Also called improper disjunctive syllogism, fallacy of the alternative syllogism.

P or Q; P is true; so Q must be false. Invalid when “or” allows both.

What it is

Reasoning from “P or Q” and “P” to “not Q”, as if confirming one option ruled out the other, when both could be true.

The pattern

P or Q. P. Therefore, not Q. (invalid) Valid look-alike (disjunctive syllogism): P or Q. Not P. Therefore, Q.

An example

ExampleAffirming a disjunct

“The car won’t start, so either the battery is dead or the starter is broken. The battery is dead, so the starter is fine.”

Both could be faulty. Replacing only the battery might not fix the car.

Why it fails

In logic, and usually in life, “or” is inclusive: at least one is true, possibly both. Confirming one option tells you nothing about the other.

When it’s not a fallacy

Most lists skip this part. Calling a fair point a fallacy is a mistake too.

It can be fair when

The options genuinely exclude each other (“exclusive or”), such as “the coin landed heads or tails”, or “you may take the train or the bus, but not both”.

Try one

A question from the app. The wrong answers are its usual look-alikes.

Practice questionFormal Logic

Gamer, on Discord: “Can’t connect again. It’s the game’s servers or my Wi-Fi, which has been cutting out all week. The status page is all red, so the servers are definitely down. That means my Wi-Fi’s fine. No need to restart the router. Guess it’s movie night.”

What’s the flaw?

Often confused with

False dilemma
A false dilemma leaves options out. Affirming a disjunct wrongly assumes that listed options can’t both hold.
Denying a conjunct
Denying a conjunct starts from “not both” and wrongly concludes one must be true.

Where the name comes from

A disjunct is one option in an “or” statement, from a Latin word for “separated.” Affirming one means saying it’s true, then wrongly ruling the other out. The disjunctive syllogism is the valid “or” argument: this option’s false, so it’s the other one. Improper disjunctive syllogism names this broken copy of it. Some logic books call an “or” that allows both an alternative, hence fallacy of the alternative syllogism.

Sources

Affirming a Disjunct, The Fallacy Files

In the app

Practice spotting it on arguments that sound real.

Steelman has more examples of affirming a disjunct, how to answer it in a real conversation, and practice rounds that mix it with its look-alikes, so you learn the difference, not just the name. Coming soon to iPhone and Android.