It is better for us not to know low and vile things, because by them we are impeded in our knowledge of what is better and higher; for we cannot understand many things simultaneously; because the thought of evil sometimes perverts the will towards evil.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil understanding
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17611/pg17611-images.html

Context

“For every operative science is the more perfect, the more it considers the particular things with which its action is concerned. Reply Obj. 2: God's immediate provision over everything does not exclude the action of secondary causes; which are the executors of His order, as was said above (Q. 19, AA. 5, 8) . Reply Obj. 3: It is better for us not to know low and vile things, because by them we are impeded in our knowledge of what is better and higher; for we cannot understand many things simultaneously; because the thought of evil sometimes perverts the will towards evil. This does not hold with God, Who sees everything simultaneously at one glance, and whose will cannot turn in the direction of evil. _______________________ FOURTH ARTICLE [I, Q. 22, Art. 4] Whether Providence Imposes Any Necessity on Things Foreseen?” source