RationalWiki's 2018 Fundraiser

There is no RationalWiki without you. We are a small non-profit with no staff — we are hundreds of volunteers who document pseudoscience and crankery around the world every day. We will never allow ads because we must remain independent. We cannot rely on big donors with corresponding big agendas. We are not the largest website around, but we believe we play an important role in defending truth and objectivity.

If everyone seeing this today donates $5, we will meet our goal for 2018.

Fighting pseudoscience isn't free.
We are 100% user-supported! Help and donate $5, $20 or whatever you can today with PayPal Logo.png!

Donations so far: $3630Goal: $5000

Existential assumption

From RationalWiki
Jump to: navigation, search
Part of the series on
Logic and rhetoric
Icon logic.svg
Key articles
General logic
Bad logic

An existential assumption (or existential fallacy) occurs whenever the conclusion of a syllogism requires that a class has at least one member, but one or more of the premises do not.

The fallacy is a syllogical fallacy and a formal fallacy.

Form[edit]

P1: All X, if they existed, would be Y. (valid)
P2: All Y that exist are Z. (valid)
C1: All X are Z. (invalid)

Explanation[edit]

Universally true premises need not be instantiated. The truth value of the statement "all trespassers will be shot" does not depend on the existence of trespassers. People can still vow to shoot at any and all trespassers, even if none currently exist on their plot of land.

The fallacy occurs when there is a mismatch between the necessity of existence for the first and second premises. For example:


P1: All unicorns are animals. (does not require existence; valid)
P2: Animals are dangerous. (requires existence; valid)
C1: Therefore, unicorns are dangerous. (invalid)

The problem with this argument is that the premises don't imply that conclusion is true, because a unicorn doesn't need to exist for it to be classed as an animal, yet it does need to exist to be dangerous.

See also[edit]

External links[edit]