Laudator Temporis Acti

Georg Christoph Lichtenberg, Sudelbücher (K 303, tr. R.J. Hollingdale):

Doubt everything at least once, even the proposition that two times two equals four.

Zweifle an allem wenigstens einmal, und wäre es auch der Satz: zweimal 2 ist 4.

A.E. Housman, Last Poems, XXXV (final stanza):

To think that two and two are four
  And neither five nor three
The heart of man has long been sore
  And long 'tis like to be.

H.L. Mencken, A Mencken Chrestomathy (New York: Knopf, 1949; rpt. New York: Random House, 1982), pp. 13-14:

A metaphysician is one who, when you remark that twice two makes four, demands to know what you mean by twice, what by two, what by makes, and what by four. For asking such questions metaphysicians are supported in oriental luxury in the universities, and respected as educated and intelligent men.

Albert Camus, The Plague:

But there always comes a time in history when the man who dares to say that two and two make four is punished with death.

Mais il vient toujours une heure dans l'histoire où celui qui ose dire que deux et deux font quatre est puni de mort.