For a man is stated to be inconstant because his reason fails in commanding what has been counselled and judged.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic reason state
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

“Consequently inconstancy, as to its completion, belongs to imprudence. And just as precipitation is due to a defect in the act of counsel, and thoughtlessness to a defect in the act of judgment, so inconstancy arises from a defect in the act of command. For a man is stated to be inconstant because his reason fails in commanding what has been counselled and judged. Reply Obj. 1: The good of prudence is shared by all the moral virtues, and accordingly perseverance in good belongs to all moral virtues, chiefly, however, to fortitude, which suffers a greater impulse to the contrary.” source