“Practice is the criterion of truth.”

Source: The Ultimate Foundation of Economic Science (1962), Chapter 5: On Some Popular Errors Concerning the Scope and Method of Economics, § 9 : The Belief in the Omnipotence of Thought

Manuscript, Sermons; reported in Hoyt's New Cyclopedia Of Practical Quotations (1922), p. 254.

"Six Asides About Culture"
Living in Truth (1986)
Context: There is only one Art, whose sole criterion is the power, the authenticity, the revelatory insight, the courage and suggestiveness with which it seeks its truth. … Thus, from the standpoint of the work and its worth it is irrelevant to which political ideas the artist as a citizen claims allegiance, which ideas he would like to serve with his work or whether he holds any such ideas at all.

Source: Language, Truth, and Logic (1936), p. 16.
Context: The criterion which we use to test the genuineness of apparent statements of fact is the criterion of verifiability. We say that a sentence is factually significant to any given person, if, and only if, he knows how to verify the proposition which it purports to express — 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.

Drawn and Quartered (1983)

Entering the Castle : An Inner Path to God and Your Soul (2007), p. 66
Context: Self-examination is the process of accountability to your soul... It is far better to "become" your truth than to speak your truth. Self-examination is the practice of becoming your truth.

Source: Nietzsche (1946), pp. 187-188
Context: For any community and those living in it, only that is true which can be communicated to all. Hence universal communicability is unconsciously accepted as the source and criterion of those truths that promote life through communal means. Truth is that which our conventional social code accepts as effective in promoting the purposes of the group. … This community will condemn as a “liar” the person who misuses its unconsciously accepted, and therefore valid, metaphors. … Community members are obliged to “lie” in accordance with fixed convention. To put it otherwise, they must be truthful by playing with the conventionally marked dice. To fail to pay in the coin of the realm is to tell forbidden lies, for, on this view, whatever transcends conventional truth is a falsehood. To tell lies of this kind is to sacrifice the world of meanings upon which the endurance of his community rests. Conversely, there are forbidden truths: This same threat to the continuance of the community is also counteracted by relentlessly preventing anyone from thinking and uttering unconventional but authentic truths.

Source: Brave New World

Source: Eclipse of Reason (1947), p. 52.