“Take advantage of all the pleasant occasions in your life, every instant that passes will be one instant less, not one more.”

Source: Walden

Source: The Year of Magical Thinking

Source: Dune

Letter (27 June 1760), published in The Works of the Rev. John Wesley (1813) Vol. XVI, p. 109
As quoted in an 1856 edition of Works
General sources
Variant: Every one, though born of God in an instant, yea, and sanctified in an instant, yet undoubtedly grows by slow degrees.

Section 60
Reflections on the Human Condition (1973)
Context: One wonders whether a generation that demands instant satisfaction of all its needs and instant solution of the world's problems will produce anything of lasting value. Such a generation, even when equipped with the most modern technology, will be essentially primitive — it will stand in awe of nature, and submit to the tutelage of medicine men.

Philosophical Essay on Probabilities (1902)
Context: Given for one instant an intelligence which could comprehend all the forces by which nature is animated and the respective situation of the beings who compose it—an intelligence sufficiently vast to submit these data to analysis—it would embrace in the same formula the movements of the greatest bodies of the universe and those of the lightest atom; for it, nothing would be uncertain and the future, as the past, would be present to its eyes. The human mind offers, in the perfection which it has been able to give to astronomy, a feeble idea of this intelligence. Its discoveries in mechanics and geometry, added to that of universal gravity, have enabled it to comprehend in the same analytical expressions the past and future states of the system of the world.<!--p.4

The Canon of Judo (1956, 1960)