“In fact, nothing is said that has not been said before.”

Book II, chapter LVIII, section 119
Cf. René Descartes' "On ne sauroit rien imaginer de si étranger et si peu croyable, qu'il n'ait été dit par quelqu'un des philosophes [One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another]" (Le Discours de la Méthode, Pt. 2)
De Divinatione – On Divination (44 BC)
Original: (la) Nihil tam absurde dici potest, quod non dicatur ab aliquo philosophorum.

Natural Theology (1836), Bk. II, Ch. III : On the Strength of the Evidences for a God in the Phenomena of Visible and External Nature, § 15; though provided without attribution of author, the saying "There is nothing more uncommon than common sense" has since become misattributed to particular people, including Frank Lloyd Wright.

True though this may have been in the past, it is true no longer. The science of our century has forged weapons of observation and analysis by which the veriest tyro may profit. Science has trained and fashioned the average mind into habits of exactitude and disciplined perception, and in so doing has fortified itself for tasks higher, wider, and incomparably more wonderful than even the wisest among our ancestors imagined. Like the souls in Plato's myth that follow the chariot of Zeus, it has ascended to a point of vision far above the earth. It is henceforth open to science to transcend all we now think we know of matter and to gain new glimpses of a profounder scheme of Cosmic law.
Address to the British Association for the Advancement of Science (1898)

Maslow (1954), as cited in: Hiram E. Fitzgerald, ‎Michael G. Walraven (1987). Psychology. p. 119; Also in: Maslow, Toward a Psychology of Being. Simon and Schuster, 1962, p. 5.
Variant quote: Human nature is not nearly as bad as it has been thought to be... It is as if Freud supplied us with the sick half of psychology and we must now fill it out with the healthy half.'
1940s-1960s

What Are Masterpieces and Why Are There So Few of Them (1936)

As quoted in "First African American Senator" http://www.senate.gov/artandhistory/history/minute/First_African_American_Senator.htm, United States Senate

On War against the Turk (1529)

This popular quote seems to be a paraphrase of an exchange in The Last Continent:
Is it true that your life passes before your eyes before you die?'
YES.
'Ghastly thought, really.' Rincewind shuddered. 'Oh, gods, I've just had another one. Suppose I am just about to die and this is my whole life passing in front of my eyes?'
I THINK PERHAPS YOU DO NOT UNDERSTAND. PEOPLE'S WHOLE LIVES DO PASS IN FRONT OF THEIR EYES BEFORE THEY DIE. THE PROCESS IS CALLED 'LIVING'.
Misc