Full Idea
Propositional language is very rudimentary and has limited powers of expression. The only relation between various statements it can handle is that of identity and difference. As are all the same, but Bs can be different from As.
Gist of Idea
Propositional language can only relate statements as the same or as different
Source
Michal Walicki (Introduction to Mathematical Logic [2012], 7 Intro)
Book Reference
Walicki,Michal: 'Introduction to Mathematical Logic' [World Scientific 2012], p.183
A Reaction
[second sentence a paraphrase] In predicate logic you could represent two statements as being the same except for one element (an object or predicate or relation or quantifier).