I answer that, To speak so as to belittle oneself may occur in two ways. First so as to safeguard truth, as when a man conceals the greater things in himself, but discovers and asserts lesser things of himself the presence of which in himself he perceives.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic truth presence
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/18755/pg18755-images.html

Context

“Therefore irony is not a sin.
On the contrary, Augustine says (De Verb. Apost., Serm. xxix) : "If thou liest on account of humility, if thou wert not a sinner before lying, thou hast become one by lying."
I answer that, To speak so as to belittle oneself may occur in two ways. First so as to safeguard truth, as when a man conceals the greater things in himself, but discovers and asserts lesser things of himself the presence of which in himself he perceives. To belittle oneself in this way does not belong to irony, nor is it a sin in respect of its genus, except through corruption of one of its circumstances. Secondly, a person belittles himself by forsaking the truth, for instance by ascribing to himself something mean the existence of which in himself he does not perceive, or by denying something great of himself, which nevertheless he perceives himself to possess: this pertains to irony, and is always a sin.
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