Full Idea
Aquinas infers from Aristotle that intellectual understanding is the only operation of the soul that is performed without a physical organ, so that only human souls, and not animal ones, can be immortal.
Gist of Idea
Humans have a non-physical faculty of reason, so they can be immortal
Source
report of Thomas Aquinas (Summa Theologicae [1265], I, q75, a3, resp) by Richard Sorabji - Rationality 'Reason'
Book Reference
'Rationality in Greek Thought', ed/tr. Frede,M /Striker,G [OUP 1999], p.329
A Reaction
This shows why so many thinkers are desperate to hang on to dualism, of some sort. Interesting that he only claims partial dualism.