“There are three methods to gaining wisdom. The first is reflection, which is the highest. The second is limitation, which is the easiest. The third is experience, which is the bitterest.”
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Confucius 269
Chinese teacher, editor, politician, and philosopher -551–-479 BCRelated quotes

The Analects, as reported in Chambers Dictionary of Quotations (1997), p. 279.
Attributed
Knowing Yourself: The True in the False (1996)
Context: You think: you become that thought. And consciousness, or the state of pure awareness, is lost. The highest knowledge man can possess is that which is true in his own experience. If his experience is limited, so is his knowledge and he behaves accordingly.

Lives of the Ten Orators
Bartlett's Familiar Quotations, 10th ed. (1919)

Herbart (1982b, p. 22), as cited in: Norbert Hilgenheger, "Johann Friedrich Herbart (1776-1841)." Kwartalnik Pedagogiczny 3-4 (1999): 5-26.
As quoted in Hans Hofmann (1963) by William Chapin Seitz, p. 15
1960s

As quoted in Diogenes Laertius, The Lives and Opinions of Eminent Philosophers, Chapter "Life of Anacharsis", 1702 edition, John Nicholson, p. 55.
Source: [Diogenes Laërtius, Diogenes_Laërtius, The Lives of the Ancient Philosophers: Containing an Account of Their Several Fects, Doctrines, Actions and Remarkable Sayings..., http://books.google.com/books?id=SQrULxU3TXMC, 4 September 2013, 1702, John Nicholson, 54, Life of Anarchasis]