“Man is not dead when he is cold, stiff, pulseless, breathless, and even showing signs of decomposition; he is not dead when buried, nor afterward, until a certain point is reached. That point is, when the vital organs have become so decomposed, that if reanimated, they could not perform their customary functions; when the mainspring and cogs of the machine, so to speak, are so eaten away by rust, that they would snap upon the turning of the key. Until that point is reached, the astral body may be caused, without miracle, to reenter its former tabernacle, either by an effort of its own will, or under the resistless impulse of the will of one who knows the potencies of nature and how to direct them. The spark is not extinguished, but only latent — latent as the fire in the flint, or the heat in the cold iron.”

Speech delivered at the Yad Vashem Museum at Jerusalem, Israel - March 23, 2000 http://www.emersonkent.com/speeches/yad_vashem.htm

Source: De potentia (c. 1265–1266) q. 7, art. 5, ad 14

Source: Think Big (1996), p. 244

Attributed to Twain but never sourced, this quotation should not be regarded as authentic.
Misattributed

Ventures in Common Sense (1919), p55.

5; variant translations:
From a certain point onward there is no longer any turning back. That is the point that must be reached.
As quoted in The Unfinished Country: A Book of American Symbols (1959) by Max Lerner, p. 452; also in Wait Without Idols (1964) by Gabriel Vahanian, p, 216; in Joyce, Decadence, and Emancipation (1995) by Vivian Heller, 39; in "The Sheltering Sky" (1949) by Paul Bowles, p. 213; and in the poem "Father and Son" by Delmore Schwartz.
There is a point of no return. This point has to be reached.
The Zürau Aphorisms (1917 - 1918)
Variant: From a certain point onward there is no longer any turning back. That is the point that must be reached.
Source: The Trial

Hagakure (c. 1716)

The Golden Fleece (1944), Invocation.
General sources

My Life in Court (1961), p. 115.