In the physical world everything is determined by causal necessity...

In the physical world everything is determined by causal necessity, and hence there is no liberty. The faculty which is not determined by causal motives declares its independence of these causes and hence is an immaterial faculty. The soul upon which such a faculty depends must be of the same nature as the faculty; that is, the soul must be immaterial.

The human soul, since it is immaterial and performs acts which are not absolutely dependent upon the bodily organs, does not perish with the body -- although, as Aquinas says,…