T5.3 2 of 4

Here is an easy rule of replacement for logically equivalent sentences.

DN
(Double Negation)
  P ~~P

Rules of replacement, the double-arrow rules, are especially powerful because they work not only on whole sentences (like the rules of inference) but also on the components of sentences.

So, for example...

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5.3a

 




Justification:       Sentence:

 

After completing the problems...