“Nitimur in vetitum" -- "We strive after the forbidden”

Source: Zen and the Art of Motorcycle Maintenance (1974), Ch. 29
Context: "What moves the Greek warrior to deeds of heroism," Kitto comments, "is not a sense of duty as we understand it—duty towards others: it is rather duty towards himself. He strives after that which we translate 'virtue' but is in Greek aretê, 'excellence' … we shall have much to say about aretê. It runs through Greek life."
There, Phædrus thinks, is a definition of Quality that had existed a thousand years before the dialecticians ever thought to put it to word-traps. Anyone who cannot understand this meaning without logical definiens and definendum and differentia is either lying or so out of touch with the common lot of humanity as to be unworthy of receiving any reply whatsoever.

Accuracy
The Note-Books of Samuel Butler (1912), Part IX - A Painter's Views on Painting

Source: Stag's Leap: Poems

Speech at Kinnaird Hall, Dundee, Scotland ("Unemployment"), October 10, 1908, in Liberalism and the Social Problem (1909), Churchill, Echo Library (2007), p. 87
Early career years (1898–1929)
Context: What is the use of living, if it be not to strive for noble causes and to make this muddled world a better place for those who will live in it after we are gone? How else can we put ourselves in harmonious relation with the great verities and consolations of the infinite and the eternal? And I avow my faith that we are marching towards better days. Humanity will not be cast down. We are going on swinging bravely forward along the grand high road and already behind the distant mountains is the promise of the sun.

1940s, Fourth Inaugural Address (1945)
Context: We shall strive for perfection. We shall not achieve it immediately — but we still shall strive. We may make mistakes — but they must never be mistakes which result from faintness of heart or abandonment of moral principle.

Signposts to Elsewhere (2008)

Source: Erewhon (1872), Ch. 19

Source: Meditations in Wall Street (1940), p. 101