Logic / Unit 5 Logic 203 short lesson, about 20 seconds
Replacement Rules: The Concept
Captions available
The idea
Replacement rules are equivalences: they swap interchangeable forms, in either direction, applied to whole lines or any part of a line.
In practice
Inference rules are one-way and whole-line; replacement rules are two-way and reach inside formulas.
Where people slip
This difference in reach is exactly why the system needs both kinds. The "::" in what follows means "replace either side with the other."
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