“Impossible gadgets are always the most valuable.”

The Innocence of Father Brown (1911) The Queer Feet
The Father Brown Mystery Series (1910 - 1927)

Source: The Purpose Driven Life: What on Earth Am I Here for?

Variant: Time is the most precious gift in our possession, for it is the most irrevocable.
Source: As quoted in LIFE magazine (22 April 1957), p. 152; also in Letters and Papers from Prison (1967), p. 47.
Context: Time is the most precious gift in our possession, for it is the most irrevocable. This is what makes it so disturbing to look back upon the time which we have lost. Time lost is time when we have not lived a full human life, time unenriched by experience, creative endeavor, enjoyment, and suffering. Time lost is time not filled, time left empty.

In a letter to Beatrice (22 August 1943), published in The Patton Papers 1940-1945 (1996) edited by Martin Blumenson https://books.google.com/books?id=eV2pRL7arKkC&pg=PT239&dq=Moral+courage+is+the+most+valuable+and+usually+the+most+absent+characteristic+in+men.&hl=en&sa=X&ved=0ahUKEwjPrbHtvsXVAhXBRyYKHUz6CAw4ChDoAQhCMAU#v=onepage&q=Moral%20courage%20is%20the%20most%20valuable%20and%20usually%20the%20most%20absent%20characteristic%20in%20men.&f=false

Majlisi, Bihārul Anwār, vol.78, p. 352.
Regarding Knowledge & Wisdom, General

Source: Principles of Economics, (1890), p. 468 (9th ed. 2009).
Context: If we compare one country of the civilized world with another, or one part of England with another, or one trade in England with another, we find that the degradation of the working-classes varies almost uniformly with the amount of rough work done by women. The most valuable of all capital is that invested in human beings; and of that capital the most precious part is the result of the care and influence of the mother, so long as she retains her tender and unselfish instincts, and has not been hardened by the strain and stress of unfeminine work.

Speech to the Canadian Club in Toronto (6 August 1927), quoted in Our Inheritance (London: Hodder and Stoughton, 1938), p. 79.
1927
Context: I may confess to men here, of a stock so largely English, that our English intelligence is sometimes apt to be despised by nations that think they are quicker-witted than we are. Our most valuable real estate is our character— its steadiness, its reliability, its personal integrity, its capacity for toleration and for a quiet, humorous boredom with things. The general strike in England, which was not without its alarming aspects, illustrated all these qualities in our people.

Diogenes Laërtius, Lives and Opinions of Eminent Philosophers, Book 5.