Full Idea
It is necessary to say that that which is the principle of intellective activity, what we call the soul of a human being, is an incorporeal, subsistent principle.
Gist of Idea
The human intellectual soul is an incorporeal, subsistent principle
Source
Thomas Aquinas (Summa Theologicae [1265], Ia.Q75 2c), quoted by Kretzmann/Stump - Aquinas, Thomas 10
Book Reference
'Shorter Routledge Encyclopaedia of Philosophy', ed/tr. Craig,Edward [Routledge 2005], p.39
A Reaction
Note 'subsistent' rather than 'existent' (capable of independence?). This identifies the immortal soul with the conscious mind. 'Principle' is an odd word, presumably with roots in Aristotle. It seems to be an Aristotelian 'form' [morphe].