A field guide to the ways reasoning breaks
Broken structure
Also called Existential instantiation error
Concluding that something exists from premises that only describe a category conditionally.
All unicorns are white, so some white things are unicorns.
Ask whether any member of the category has been shown to exist.
A fallacy of the quantified syllogism: premises that only say every member of a class would have a property are read as proving that some member exists. Medieval logic assumed universal statements carried 'existential import'; Bertrand Russell's 1905 essay 'The Existential Import of Proposition' helped settle the modern, Boolean convention that they do not.
All unicorn sightings in the data are fake; therefore some fake sightings are unicorn sightings — the set may simply be empty.
Every overdue invoice gets a penalty fee; therefore some invoices have penalty fees — says the report whose penalty table has zero rows.
All prohibited items are confiscated at the gate; so some items were confiscated today — on a day nothing was prohibited from entering.
All ghost sightings happen at night; therefore some nighttime events are ghost sightings — before establishing a single sighting exists.