“Custom is almost a second nature.”
Rules for the Preservation of Health, 18
Bartlett's Familiar Quotations, 10th ed. (1919)
Plutarch , later named, upon becoming a Roman citizen, Lucius Mestrius Plutarchus, was a Greek biographer and essayist, known primarily for his Parallel Lives and Moralia.
He is classified as a Middle Platonist. Plutarch's surviving works were written in Greek, but intended for both Greek and Roman readers. Wikipedia

“Custom is almost a second nature.”
Rules for the Preservation of Health, 18
Bartlett's Familiar Quotations, 10th ed. (1919)
Life of Coriolanus
Bartlett's Familiar Quotations, 10th ed. (1919)
Symposiacs, book viii. Question viii
Bartlett's Familiar Quotations, 10th ed. (1919)
Of Pausanias the Son of Phistoanax
Laconic Apophthegms
Plutarch book Parallel Lives
Gaius Marcius (Coriolanus) 14.2, translated by Ian Scott-Kilvert, Makers of Rome: Nine Lives by Plutarch (Harmondsworth : Penguin Books 1965) ISBN 0140441581, p. 27
Parallel Lives
57 Lycurgus
Apophthegms of Kings and Great Commanders
“Like watermen, who look astern while they row the boat ahead.”
Whether 't was rightfully said, Live Concealed
Bartlett's Familiar Quotations, 10th ed. (1919)
Why the Oracles cease to give Answers
Bartlett's Familiar Quotations, 10th ed. (1919)
“According to the proverb, the best things are the most difficult.”
Moralia, Of the Training of Children
“The measure of a man's life is the well spending of it, and not the length.”
Consolation to Apollonius
“Perish those who suspect those men of doing or enduring anything base.”
Plutarch book Parallel Lives
Pelopidas, sec. 18
Parallel Lives
Plutarch book Parallel Lives
Marcus Cato, sec. 19
Parallel Lives
Plutarch book Parallel Lives
Marcus Cato, sec. 7
Parallel Lives
Plutarch book Parallel Lives
Marcus Cato, sec. 4
Parallel Lives
Plutarch book Parallel Lives
Demosthenes, sec. 16
Parallel Lives
Plutarch book Parallel Lives
Paulus Aemilius and Timeleon, sec. 2
Parallel Lives
Plutarch book Parallel Lives
Aemilius, sec. 36
Parallel Lives
Plutarch book Parallel Lives
Aemilius, sec. 27
Parallel Lives
II, 4
Moralia, Of Eating of Flesh
I, 4
Moralia, Of Eating of Flesh
Source: Moralia, How one may be aware of one's progress in virtue, XV
“Impossible questions require impossible answers.”
Plutarch book Parallel Lives
Alexander, sec. 54
Parallel Lives
“I myself had rather excel others in excellency of learning than in greatness of power.”
Plutarch book Parallel Lives
Alexander, sec. 7
Parallel Lives
“But those who are careless of accuracy in small things soon begin to neglect the most important.”
Plutarch book Parallel Lives
Aemilius, sec. 3
Parallel Lives