Again, a human act is evil through lacking conformity with its due measure: and conformity of measure in a thing depends on a rule, from which if that thing depart, it is incommensurate.
 Thomas Aquinas, Summa Theologica (1274). copy citation

add
Author Thomas Aquinas
Source Summa Theologica
Topic conformity evil
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

“Now that an act is a human act is due to its being voluntary, as stated above (Q. 1, A. 1) , whether it be voluntary, as being elicited by the will, e.g. to will or to choose, or as being commanded by the will, e.g. the exterior actions of speech or operation. Again, a human act is evil through lacking conformity with its due measure: and conformity of measure in a thing depends on a rule, from which if that thing depart, it is incommensurate. Now there are two rules of the human will: one is proximate and homogeneous, viz. the human reason; the other is the first rule, viz. the eternal law, which is God's reason, so to speak. Accordingly Augustine (Contra Faust. xxii, 27) includes two things in the definition of sin; one, pertaining to the substance of a human act, and which is the matter, so to speak, of sin, when he says "word, deed, or desire"; the other, pertaining to the nature of evil, and which is the form, as it were, of sin, when he says, "contrary to the eternal law."
source