The criterion which we use to test the genuineness of apparent statements of fact is the criterion of verifiability.We say that a sentence isfactually significant to any given person, if, and only if, he knows how to 44 verify the proposition which it purports to express ö that is, if he knows what observations would lead him, under certain conditions, to accept the proposition as being true, or reject it as being false.

-Ayer, SirAlfred Jules
  Language, Truth and Logic, ch.1

It is perfectly true, as philosophers say, that life must be understood backwards.But they forget the other proposition, that it must be lived forwards. And if one thinks over that proposition it becomes more and more evident that life can never really be understood in time simply because at no particular moment can I find the necessary resting-place from which to understand itöbackwards.

-Kierkegaard, So«  ren Aabye
Journal entry (translated by Alexander Dru,1938).

Doubt everything at least once, even the proposition that two times two equals four.

-Lichtenberg, Georg Christoph
c.1796  Aphorisms, Notebook K (translated by R  J Hollingdale, 1990).

Four score and sevenyears ago our fathers brought forth upon this continent a new nation, conceived in liberty and dedicated to the proposition that all men are created equal†we here highly resolve that the dead shall not have died in vain; that this nation, under God, shall have a new birth of freedom; and that government of the people, by the people, and for the people, shall not perish from the earth. 510

-Lincoln, Abraham
  Dedication address, Gettysburg NationalCemetery,19 Nov.

Many politicians of our time are in the habit of laying it Macaulay down as a self-evident proposition, that no people ought to be free till theyare fit to use their freedom. The maxim is worthy of the fool in the old story, who resolved not to go into the water till he had learnt to swim. If men are to wait for liberty till they become wise and good in slavery, they may indeed wait for ever.

-1st Baron
  'Milton', in the Edinburgh Review,  Aug.

Every proposition which we can understand must be composed wholly of constituents with which we are acquainted.

-Russell, Bertrand Arthur William Russell, 3rd Earl
  A Free Man'sWorship,'Knowledge byAcquaintance and Knowledge by Description'.

Where God's presence is no longer a tenable proposition and where his absence is no longer a felt, indeed overwhelming weight, certain dimensions of thought and creativity are no longer attainable.

-Steiner, George
  Real Presences.