“RADICAL—one who goes too far.
CONSERVATIVE—one who does not go far enough.
REACTIONARY—one who does not go at all.”

Sie muss also zu weit gehen, um herauszufinden, wie weit sie gehen darf.
"Die Freiheit der Kunst", speech delivered at Wuppertal on September 24, 1966; cited from Cultura 21 magazine http://www.cultura21.de/magazin/denkanstosse/d20050930a.html, September 30, 2005. Translation: Walter Laqueur Germany Today: A Personal Report (London: Weidenfeld and Nicolson, 1985) p. 130.

Martí : Thoughts/Pensamientos (1994)

Letter to Carl Friedrich Zelter (3 December 1812)

Source: A History of Experimental Psychology, 1929, p. 397: Cited in: Jay M. Jackson (2013) Social Psychology, Past and Present: An Integrative Orientation, p. 28

As quoted in The Modern Handbook of Humor (1967) by Ralph Louis Woods
Variants:
A conservative is someone who admires radicals a century after they're dead.
A conservative is one who admires radicals centuries after they're dead.

[Lumley, Frederick, New Trends in 20th Century Drama: A Survey Since Ibsen and Shaw, Barrie and Jenkins, 1972, London, 12, 978-0-19-519680-1]

XI, 26, Parts of this passage has been heavily compared with later statements of René Descartes; in Latin and with a variant translations:
The City of God (early 400s)
Context: We both are, and know that we are, and delight in our being, and our knowledge of it. Moreover, in these three things no true-seeming illusion disturbs us; for we do not come into contact with these by some bodily sense, as we perceive the things outside of us of all which sensible objects it is the images resembling them, but not themselves which we perceive in the mind and hold in the memory, and which excite us to desire the objects. But, without any delusive representation of images or phantasms, I am most certain that I am, and that I know and delight in this. In respect of these truths, I am not at all afraid of the arguments of the Academicians, who say, What if you are deceived? For if I am deceived, I am. For he who is not, cannot be deceived; and if I am deceived, by this same token I am. And since I am if I am deceived, how am I deceived in believing that I am? for it is certain that I am if I am deceived. Since, therefore, I, the person deceived, should be, even if I were deceived, certainly I am not deceived in this knowledge that I am. And, consequently, neither am I deceived in knowing that I know. For, as I know that I am, so I know this also, that I know. And when I love these two things, I add to them a certain third thing, namely, my love, which is of equal moment. For neither am I deceived in this, that I love, since in those things which I love I am not deceived; though even if these were false, it would still be true that I loved false things. For how could I justly be blamed and prohibited from loving false things, if it were false that I loved them? But, since they are true and real, who doubts that when they are loved, the love of them is itself true and real? Further, as there is no one who does not wish to be happy, so there is no one who does not wish [themself] to be [into being]. For how can he be happy, if he is nothing?

Riyadh-as-Saliheen by Imam Al-Nawawi, volume 4, hadith number 537
Sunni Hadith

Original: (it) L'eccessiva gelosia è l'alibi di chi non dà abbastanza valore a colui che chiama amore.
Source: prevale.net