I answer that, When anything is said to be a medium in the assumption of the Incarnation, we do not signify order of time, because the assumption of the whole and the parts was simultaneous. For it has been shown (Articles [3],4) that the soul and body were mutually united at the same time in order to constitute the human nature of the Word. But it is order of nature that is signified. Hence by what is prior in nature, that is assumed which is posterior in nature. Now a thing is prior in nature in two ways: First on the part of the agent, secondly on the part of the matter; for these two causes precede the thing. On the part of the agent---that is simply first, which is first included in his intention; but that is relatively first, with which his operation begins---and this because the intention is prior to the operation. On the part of the matter---that is first which exists first in the transmutation of the matter. Now in the Incarnation the order depending on the agent must be particularly considered, because, as Augustine says (Ep. ad Volusianum cxxxvii), "in such things the whole reason of the deed is the power of the doer." But it is manifest that, according to the intention of the doer, what is complete is prior to what is incomplete, and, consequently, the whole to the parts. Hence it must be said that the Word of God assumed the parts of human nature, through the medium of the whole; for even as He assumed the body on account of its relation to the rational soul, so likewise He assumed a body and soul on account of their relation to human nature.