Logic / Unit 4 Logic 126 short lesson, about 20 seconds
Translating "Only If"
Captions available
The idea
"p only if q" means p ⊃ q — the "only if" clause is the consequent.
In practice
"You may board only if you hold a ticket" — Board ⊃ Ticket. Not the reverse: a ticket doesn't guarantee boarding.
Where people slip
This is video 089's trap wearing propositional clothes. "Only" reverses expectations in both systems.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Something wrong in this lesson? Flag it · Sources for Logic