Soul and body make, for Thomas, as single unified substance.  But the demonstration of this point depends on whether one looks at the issue from the perpective of the parts (soul/body, or body parts) or the whole.  As Robert Pasnau ( Thomas Aquinas on Human Nature: A Philosophical Study of Summa Theologiae, 1a 75-89 ) points out, from the perspective of the whole the body is separable from the person: “We can lose legs and arms, organs and tissue without the person’s being destroyed.  Moreover, we lose and gain cells on a constant basis: liver cells are replaced within five days.”

Thomas examines the question from the opposite angle, from the perspective of the parts:

“he asks whether the parts can survive without the whole.  In the case of a living substance, they cannot.  The cells of the liver cease to exist when they pass out of the body.  They no longer function as they once did, contributing in a certain way to a living organism.  From the perspective of the parts, then, the substance is an indivisible whole.”  This “explains the unity of soul and body in a way that allows for the soul’s peculiar autonomy relative to the body.”  The unity of the organism is analyzed from one direction: “the substance and its parts cannot exist without the form, but it remains possible for the form to exist without the substance.”

Thomas draws some crucial conclusions from this: “it is entirely consistent for the rational soul to be united with the body and yet to be able to operate and exist independently of the body”; plus, “the soul can be united to the body without itself becoming material.”  The soul can exist without body, but the relation is not reversible: “the body remains inseparable from the soul, in that it cannot exist without the soul.”