“Despair is typical of those who do not understand the causes of evil, see no way out, and are incapable of struggle.”

Mathematical Methods in Science (1977)
Context: We wish to see... the typical attitude of the scientist who uses mathematics to understand the world around us.... In the solution of a problem... there are typically three phases. The first phase is entirely or almost entirely a matter of physics; the third, a matter of mathematics; and the intermediate phase, a transition from physics to mathematics. The first phase is the formulation of the physical hypothesis or conjecture; the second, its translation into equations; the third, the solution of the equations. Each phase calls for a different kind of work and demands a different attitude.<!--p.164

Pearls of Wisdom

La Tontine (1709)

General Security: The Liquidation of Opium (1925)

Homebody (1998)

Book III, Ch. 5. Upon some Verses of Virgil
Essais (1595), Bartlett's Familiar Quotations, 10th ed. (1919)

This and many similar quotes with the same general meaning are misattributed to Aristotle as a result of Twitter attribution decay. The original source of the quote remains anonymous. The oldest reference resides in the works of George Bernard Shaw, Man and Superman (1903): "Maxims for Revolutionists", where he claims that “He who can, does. He who cannot, teaches.”. However, the related quote, "Those who can, do. Those who understand, teach" likely originates from Lee Shulman in his explanation of Aristotlean views on professional mastery: Source: Shulman, L. S. (1986). Those who understand: Knowledge growth in teaching. Educational Researcher, 15(2), 4 - 14. Stable URL: http://www.jstor.org/stable/1175860
Misattributed
Variant: Those who can, do, those who cannot, teach.

Book III, Ch. 5
Essais (1595), Book III