The body is not of the essence of the soul; but the soul by the nature of its essence can be united to the body, so that, properly speaking, not the soul alone, but the "composite," is the species.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic species body
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

“Thus, however, the intellectual faculty is not the noblest, because it is indeterminate and common to many degrees of intellectuality; as the sensible faculty is common to many degrees in the sensible nature. Hence, as all sensible things are not of one species, so neither are all intellectual things of one species. Reply Obj. 3: The body is not of the essence of the soul; but the soul by the nature of its essence can be united to the body, so that, properly speaking, not the soul alone, but the "composite," is the species. And the very fact that the soul in a certain way requires the body for its operation, proves that the soul is endowed with a grade of intellectuality inferior to that of an angel, who is not united to a body.” source