Full Idea
A truth-table test is entirely mechanical, ..and in propositional logic we can even generate proofs mechanically for tautological sequences, ..but this mechanical approach breaks down with predicate calculus, and proof-discovery is an imaginative process.
Gist of Idea
A truth-table test is entirely mechanical, but this won't work for more complex logic
Source
E.J. Lemmon (Beginning Logic [1965], 2.5)
Book Reference
Lemmon,E.J.: 'Beginning Logic' [Nelson 1979], p.91