Propositional Logic is Too Weak a Representational Language

Propositional Logic (PL) is not a very expressive language because:

Hard to identify "individuals." E.g., Mary, 3
Can't directly talk about properties of individuals or relations between individuals. E.g., tall(Bill)
Generalizations, patterns, regularities can't easily be represented. E.g., all triangles have 3 sides

Consider the problem of representing the following information:

Every person is mortal.
Confucius is a person.
Confucius is mortal.

How can these sentences be represented so that we can infer the third sentence from the first two? In PL we have to create propositional symbols to stand for all or part of each sentence. For example, we might do:

P = "person"
Q = "mortal"
R = "Confucius"

so the above 3 sentences are represented as:

P => Q
R => P
R => Q

and though in this case the third sentence is entailed by the first two, we needed an explicit symbol, R, to represent an individual, Confucius, who is a member of the classes "person" and "mortal." So, to represent other individuals we must introduce separate symbols for each one, with means for representing the fact that all individuals who are "people" are also "mortal." First-Order Logic (abbreviated FOL or FOPC) is expressive enough to concisely represent this kind of situation.