“The easiest thing in the world is self-deceit; for every man believes what he wishes, though the reality is often different.”
Third Olynthiac http://books.google.com/books?id=n4INAAAAYAAJ&q="the+easiest+thing+in+the+world+is+self-deceit+for+every+man+believes+what+he+wishes+though+the+reality+is+often+different"&pg=PA57#v=onepage, section 19 (349 BC), as translated by Charles Rann Kennedy (1852)
Variants:
A man is his own easiest dupe, for what he wishes to be true he generally believes to be true.
As quoted in The Routledge Dictionary of Quotations (1987) by Robert Andrews, p. 255
There is nothing easier than self-delusion. Since what man desires, is the first thing he believes.
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Démosthenés 11
ancient greek statesman and orator -384–-322 BCRelated quotes

Don Soderquist “ Live Learn Lead to Make a Difference https://books.google.com/books?id=s0q7mZf9oDkC&lpg=pg=PP1&dq=Don%20Soderquist&pg=PP1#v=onepage&q&f=false, Thomas Nelson, April 2006 p. 87.
On Listening

“Man is naturally deceitful ever, in every way! ”
(tr. Hickie 1853, vol. 1, p. 326 http://books.google.com/books?id=Cm4NAAAAYAAJ&pg=PA326)
Birds (414 BC)
Variant: Man naturally is deceitful, ever indeed, and always, in every one thing.

Source: Think and Grow Rich: The Landmark Bestseller - Now Revised and Updated for the 21st Century

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 319.

The Tragic Sense of Life (1913), X : Religion, the Mythology of the Beyond and the Apocatastasis
“Falling in love should be the easiest thing in the world, but it's not.”
Source: Full Moon

Preface, Hot Afternoons Have Been in Montana: Poems, 1957