xy

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

“Successful people never worry about what others are doing.”

Plato (-427–-347 BC) Classical Greek philosopher

Alleged source in Plato unknown. Earliest occurrence to have been located is a Tweet from 2011 https://twitter.com/ochocinco/status/93332058864238592. Disputed

“Ignorance, the root and stem of all evil.”

Plato (-427–-347 BC) Classical Greek philosopher

Attributed to Plato on quotes sites but never sourced.
Disputed

“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”

Plato (-427–-347 BC) Classical Greek philosopher

This quotation, often attributed on the Internet to Plato, cannot be found in any of Plato's writings, nor can it be found in any published work anywhere until recent years. If it really were a quotation by Plato, then some author in the recorded literature of the last several centuries would have mentioned that quote, but they did not. The sentiment isn't new, however. The ancient Roman Seneca, in his work on "Morals," quoted an earlier Roman writer, Lucretius (who wrote about the year 50 B.C.), as saying "we are as much afraid in the light as children in the dark." (Seneca was paraphrasing a longer passage by Lucretius from De Rerum Natura (On the Nature of Things), Book II, lines 56 et seq.)
Misattributed

“There is no one who ever acts honestly in the administration of states, nor any helper who will save any one who maintains the cause of the just.”

Plato The Republic

496d
The Republic

“Music is a moral law. It gives a soul to the universe, wings to the mind, flight to the imagination, a charm to sadness, gaiety and life to everything. It is the essence of order, and leads to all that is good, just, and beautiful, of which it is the invisible, but nevertheless dazzling, passionate, and eternal form.”

Plato (-427–-347 BC) Classical Greek philosopher

This quotation is not known to exist in Plato's writings. It apparently first appeared as a quotation attributed to Plato in The Pleasures of Life, Part II by Sir John Lubbock (Macmillan and Company, London and New York), published in 1889.
Misattributed

“All that is said by any of us can only be imitation and representation.”

Plato (-427–-347 BC) Classical Greek philosopher

107b
Critias

“The inexperienced in wisdom and virtue, ever occupied with feasting and such, are carried downward, and there, as is fitting, they wander their whole life long, neither ever looking upward to the truth above them nor rising toward it, nor tasting pure and lasting pleasures. Like cattle, always looking downward with their heads bent toward the ground and the banquet tables, they feed, fatten, and fornicate. In order to increase their possessions they kick and butt with horns and hoofs of steel and kill each other, insatiable as they are.”

Plato The Republic

586a–b
The Republic

“Just as things in a picture, when viewed from a distance, appear to be all in one and the same condition and alike.”

Plato book Parmenides

165c
Parmenides

“After much effort, as names, definitions, sights, and other data of sense, are brought into contact and friction one with another, in the course of scrutiny and kindly testing by men who proceed by question and answer without ill will, with a sudden flash there shines forth understanding about every problem, and an intelligence whose efforts reach the furthest limits of human powers.”

Plato (-427–-347 BC) Classical Greek philosopher

Online http://classics.mit.edu/Plato/seventh_letter.html The 7th Epistle