“Nothing is more certain than death and nothing uncertain but its hour.”

Source: Guilty Pleasures

Variant: It must be considered that there is nothing more difficult to carry out, nor more doubtful of success, nor more dangerous to handle, than to initiate a new order of things.
Source: The Prince (1513), Ch. 6
Context: It ought to be remembered that there is nothing more difficult to take in hand, more perilous to conduct, or more uncertain in its success, than to take the lead in the introduction of a new order of things. Because the innovator has for enemies all those who have done well under the old conditions, and lukewarm defenders in those who may do well under the new. This coolness arises partly from fear of the opponents, who have the laws on their side, and partly from the incredulity of men, who do not readily believe in new things until they have had a long experience of them.

Source: The Last Messiah (1933), To Be a Human Being https://www.youtube.com/watch?v=u4m6vvaY-Wo&t=1110s (1989–90)

Starting from Paumanok. 12
Bartlett's Familiar Quotations, 10th ed. (1919)

Source: Paradoxes of Faith (1987), Ch. X. "Man", p. 137

Late Night with David Letterman (5 March 1993).

Source: 1910s, Why Men Fight https://en.wikisource.org/wiki/Why_Men_Fight (1917), pp. 178-179
Context: Men fear thought as they fear nothing else on earth – more than ruin, more even than death. Thought is subversive and revolutionary, destructive and terrible; thought is merciless to privilege, established institutions, and comfortable habits; thought is anarchic and lawless, indifferent to authority, careless of the well-tried wisdom of the ages. Thought looks into the pit of hell and is not afraid. It sees man, a feeble speck, surrounded by unfathomable depths of silence; yet it bears itself proudly, as unmoved as if it were lord of the universe. Thought is great and swift and free, the light of the world, and the chief glory of man.