“No state sorrier than that of the man who keeps up a continual round, and pries into "the secrets of the nether world," as saith the poet, and is curious in conjecture of what is in his neighbour's heart.”
II, 13
Meditations (c. 121–180 AD), Book II
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Marcus Aurelius 400
Emperor of Ancient Rome 121–180Related quotes
The Dietetics of the Soul; Or, True Mental Discipline (1838)

Source: Blood Meridian: Or the Evening Redness in the West

“The most difficult secret for a man to keep is his own opinion of himself.”

Romans 3:19-31
Epistle to the Romans
Context: Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God. Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets; Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: For all have sinned, and come short of the glory of God; Being justified freely by his grace through the redemption that is in Christ Jesus: Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith. Therefore we conclude that a man is justified by faith without the deeds of the law. Is he the God of the Jews only? is he not also of the Gentiles? Yes, of the Gentiles also: Seeing it is one God, which shall justify the circumcision by faith, and uncircumcision through faith.
Do we then make void the law through faith? God forbid: yea, we establish the law.

“A man sees in the world what he carries in his heart.”

The Art Eternal, New York Evening Mail (1918)
1910s

“The aphorism "as a man thinketh in his heart so is he" contains the secret of life.”
Source: Striking Thoughts (2000), p. 4; Lee here quotes Proverbs 23:7 "As he thinketh in his heart, so is he."