“Play on, mortal. Every god falls at a mortal’s hands. Such is the only end to immortality.”

As quoted in Lives and Opinions of Eminent Philosophers, Sect. 30, as translated by Robert Drew Hicks (1925); also in The Demon and the Quantum: From the Pythagorean Mystics to Maxwell's Demon (2007) by Robert J. Scully, Marlan O. Scully, p. 11

As quoted in Plato's Gorgias, 484b.

Autobiography of Values (1978)
Context: I know myself as mortal, but this raises the question: "What is I?" Am I an individual, or am I an evolving life stream composed of countless selves? … As one identity, I was born in AD 1902. But as AD twentieth-century man, I am billions of years old. The life I consider as myself has existed though past eons with unbroken continuity. Individuals are custodians of the life stream — temporal manifestations of far greater being, forming from and returning to their essence like so many dreams. … I recall standing on the edge of a deep valley in the Hawaiian island of Maui, thinking that the life stream is like a mountain river — springing from hidden sources, born out of the earth, touched by stars, merging, blending, evolving in the shape momentarily seen. It is molecules probing through time, found smooth-flowing, adjusted to shaped and shaping banks, roiled by rocks and tree trunks — composed again. Now it ends, apparently, at a lava brink, a precipitous fall.
Near the fall's brink, I saw death as death cannot be seen. I stared at the very end of life, and at life that forms beyond, at the fact of immortality. Dark water bent, broke, disintegrated, transformed to apparition — a tall, stately ghost soul emerged from body, and the finite individuality of the whole becomes the infinite individuality of particles. Mist drifted, disappeared in air, a vanishing of spirit. Far below in the valley, I saw another river, reincarnated from the first, its particles reorganized to form a second body. It carried the same name. It was similar in appearance. It also ended at a lava brink. Flow followed fall, and fall followed flow as I descended the mountainside. The river was mortal and immortal as life, as becoming.

Source: Prometheus Rising (1983), Ch. 1 : The Thinker & The Prover, p. 25
Context: Comparative religion and philosophy show that the Thinker can regard itself as mortal, as immortal, as both mortal and immortal (the reincarnation model) or even as non-existent (Buddhism). It can think itself into living in a Christian universe, a Marxist universe, a scientific-relativistic universe, or a Nazi universe—among many possibilities.
As psychiatrists and psychologists have often observed (much to the chagrin of their medical colleagues), the Thinker can think itself sick, and can even think itself well again.
The Prover is a much simpler mechanism. It operates on one law only: Whatever the Thinker thinks, the Prover proves.
To cite a notorious example which unleashed incredible horrors earlier in this century, if the Thinker thinks that all Jews are rich, the Prover will prove it. It will find evidence that the poorest Jew in the most run-down ghetto has hidden money somewhere.

Verse.

"Looking Death in the Face", Miss Mulock's Poems (1866)

The Second Part, Chapter 17, p. 87 (See also: Ten Commandments).
Leviathan (1651)
Context: I Authorize and give up my Right of Governing my selfe, to this Man, or to his Assembly of men, on this condition, that thou that give up thy Right to him, and Authorise all his Actionsin like manner. This done, the Multitude so united in one Person, is called a COMMON-WEALTH, in latine CIVITAS. This is the Generation of that LEVIATHAN, or rather (to speake more reverently)of that Mortall God, to which we owe under the Immortal God, our peace and defence.

Source: The Son of Neptune

New Ideas
The Note-Books of Samuel Butler (1912), Part VII - On the Making of Music, Pictures, and Books