I answer that, Reason is the first principle of all human acts; and whatever other principles of human acts may be found, they obey reason somewhat, but in various ways.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic reason principles
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“But intellectual virtue is included in the definition of moral virtue: for the Philosopher says (Ethic. ii, 6) that "moral virtue is a habit of choosing the mean appointed by reason as a prudent man would appoint it." Now this right reason that fixes the mean of moral virtue, belongs to an intellectual virtue, as stated in Ethic. vi, 13. Therefore moral virtue does not differ from intellectual.
On the contrary, It is stated in Ethic. i, 13 that "there are two kinds of virtue: some we call intellectual; some moral."
I answer that, Reason is the first principle of all human acts; and whatever other principles of human acts may be found, they obey reason somewhat, but in various ways. For some obey reason blindly and without any contradiction whatever: such are the limbs of the body, provided they be in a healthy condition, for as soon as reason commands, the hand or the foot proceeds to action.” source