The sensitive soul is not a perfect self-subsistent substance.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic soul sensitive
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

“And as it matters not whether we say that something is moved by the instrument or by the principal agent, so neither does it matter whether we say that the soul of the generated is caused by the soul of the generator, or by some seminal power derived therefrom. Reply Obj. 1: The sensitive soul is not a perfect self-subsistent substance. We have said enough (Q. 25, A. 3) on this point, nor need we repeat it here. Reply Obj. 2: The generating power begets not only by its own virtue but by that of the whole soul, of which it is a power.” source