The early 20th century American social critic and humorist H. L. Mencken, known for his "definitions" of terms, defined a demagogue as "one who will preach doctrines he knows to be untrue to men he knows to be idiots."
Though this definition emphasizes the use of lying and falsehoods, some point out that demagogy does not require such, but that skilled demagogues often need to use only special emphasis by which an uncritical listener will be led to draw the desired conclusion himself, seeding a belief that is self-reinforced rather than one based on fact or truth.
Demagogues generally use logical fallacies, though persuasion may require no use of logic. While it may not rely heavily upon outright lies, the use of half-truths, omissions, and distortions are what define demagogy — it is, in essence, giving bad-faith arguments for the purpose of political or personal gain.
