“No one should be discouraged, Theaetetus, who can make constant progress, even though it be slow.”
Original Greek, from Sophist 261b http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0172%3Atext%3DSoph.%3Asection%3D261b: θαρρεῖν, ὦ Θεαίτητε, χρὴ τὸν καὶ σμικρόν τι δυνάμενον εἰς τὸ πρόσθεν ἀεὶ προϊέναι.
Also quoted in variant forms such as: Never discourage anyone who continually makes progress, no matter how slow
Sophist
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Plato 80
Classical Greek philosopher -427–-347 BCRelated quotes

https://theosophy.world/sites/default/files/ebooks/Annie%20Besant-In-The-Outer-Court.pdf In the Outer Court, 1895, p. 11

“Be slow to fall into friendship; but when thou art in, continue firm and constant.”

“The materials of action are variable, but the use we make of them should be constant.”
How Nobleness of Mind may be consistent with Prudence, Chap. v.
Bartlett's Familiar Quotations, 10th ed. (1919)

“Progress is not an illusion; it happens, but it is slow and invariably disappointing.”

“Change is inevitable. In a progressive country change is constant. ”

Speech delivered at the London Institute of Petroleum http://web.archive.org/web/20000414054656/http://www.petroleum.co.uk/speeches.htm, 1999
1990s

Conversation: Elon Musk on Wired Science (2007)

translation from the original Dutch, Fons Heijnsbroek
Er is in wat ik maak geen waarborg, dat ik vooruit zal gaan.. ..terwijl beter geschoolde lui, ook al lijkt hun werk op het ogenblik niet zoo goed, veel eer, verder kunnen komen.
Breitner, quoted by Jan Veth, in Portretstudies en silhouetten, J. Veth; Amsterdam 1908, p. 204
Jan Veth is remembering Breitner's remark from an earlier walk they made together
1900 - 1923