Only two things are infinite, the universe and human stupidity, and I'm not sure about the former.

Albert Einstein (1879 - 1955)

Genius may have its limitations, but stupidity is not thus handicapped.

Elbert Hubbard (1856 - 1915)

Talk sense to a fool and he calls you foolish.

Euripides (484 BC - 406 BC), The Bacchae, circa 407 B.C.

Get all the fools on your side and you can be elected to anything.

Frank Dane

Against stupidity the gods themselves contend in vain.

Friedrich von Schiller (1759 - 1805)

It is dangerous to be sincere unless you are also stupid.

George Bernard Shaw (1856 - 1950), Man and Superman (1903) "Maxims¹ for Revolutionists"

To be stupid, selfish, and have good health are three requirements for happiness, though if stupidity is lacking, all is lost.

Gustave Flaubert (1821 - 1880)

The two most common elements in the universe are Hydrogen and stupidity.

Harlan Ellison (1934 - )

Ordinarily he was insane, but he had lucid² moments when he was merely stupid.

Heinrich Heine (1797 - 1856)

There are more fools in the world than there are people.

Heinrich Heine (1797 - 1856)

There is nothing worse than aggressive stupidity.

Johann Wolfgang von Goethe (1749 - 1832)

Insanity³ is just what we call stupidity when it doesn't make sense.

Josh Lieb, I am a Genius of Unspeakable Evil and I Want to be Your Class President, 2009

Nothing in all the world is more dangerous than sincere ignorance and conscientious⁴ stupidity.

Martin Luther King Jr. (1929 - 1968), Strength to Love, 1963

If there are no stupid questions, then what kind of questions do stupid people ask? Do they get smart just in time to ask questions?

Scott Adams (1957 - )

Strange as it seems, no amount of learning can cure stupidity, and higher education positively⁵ fortifies⁶ it.

Stephen Vizinczey, An Innocent Millionaire

Artificial Intelligence is no match for natural stupidity.

Unknown

Everyone is entitled to be stupid, but some abuse the privilege.

Unknown

Fools rush in where fools have been before.

Unknown

Never attribute to malice⁷ what can be adequately explained by stupidity.

Unknown, Hanlon's Razor

To succeed in the world it is not enough to be stupid, you must also be well-mannered.

Voltaire (1694 - 1778)

Lord, what fools these mortals be!

William Shakespeare (1564 - 1616), "A Midsummer Night's Dream", Act 3 scene 2