“5451. We never know the Worth of Water, till the Well is dry.”
Introductio ad prudentiam: Part II (1727), Gnomologia (1732)
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Thomas Fuller (writer) 420
British physician, preacher, and intellectual 1654–1734Related quotes

In Search of a Better World (1984)
Context: There are uncertain truths — even true statements that we may take to be false — but there are no uncertain certainties.
Since we can never know anything for sure, it is simply not worth searching for certainty; but it is well worth searching for truth; and we do this chiefly by searching for mistakes, so that we have to correct them.

“We never do anything well till we cease to think about the manner of doing it.”
"On Prejudice"
Men and Manners: Sketches and Essays (1852)

“We may never come to full knowing of God till we know first clearly our own Soul.”
Summations, Chapter 56
Variant: We can never come to full knowing of God till we know first clearly our own Soul.

“But toms will till. I know he well.”
Book I, Chapter 8
'time will tell'; 'I know he will / I know him well'
Finnegans Wake (1939)

“A sincere diplomat is like dry water or wooden iron.”
Speech "The Elections in St. Petersburg" (January 1913) http://marx2mao.com/Stalin/ESP13.html
Stalin's speeches, writings and authorised interviews

“We seem never to know what any thing means or is worth until we have lost it.”
Source: Morals and Dogma of the Ancient and Accepted Scottish Rite of Freemasonry (1871), Ch. XXII : Grand Master Architect, p. 190

Reported in Alpheus Thomas Mason, Harlan Fiske Stone, Pillar of the Law (1956), p. 731; Mason reports this as a toast Stone was fond of reciting, but does not settle authorship with Stone. Various other sources following Mason attribute authorship to Stone, but without citing an original source.
Attributed

Said on a visit to Lockerbie in 1993 to a man who lived in a road where eleven people had been killed by wreckage from the Pan Am jumbo jet, as quoted in "Prince Philip's gaffes" http://news.bbc.co.uk/1/hi/uk/416992.stm, BBC News (10 August 1999)
1990s