Of God and His Creatures4. The body which the Word assumed was formed from the first instant of conception, because it would have been against the fitness of things for the Word of God to have assumed anything that was formless. Moreover the soul, like any other natural form, requires its proper matter. Now the proper matter of the soul is an organised body: for "the soul is the actualisation of an organic, natural body, that is in potentiality to life."* If then the soul was united with the body from the first instant of conception, the body must needs have been organised and formed from the first instant of conception. Moreover in the order of the stages of generation the organisation of the body precedes the introduction of the rational soul: hence, positing the latter, we must posit the former stage also. But increase in quantity up to the due measure may very well be subsequent to the animation of the body. Thus then, concerning the conception of the Man assumed, we must think that in the very instant of conception His body was organised and formed, but had not as yet its due quantity.*
4.40, 49 : Objections against the Faith of the Incarnation, with Replies
4.45 : That Christ was born of a Virgin without prejudice to His true and natural Humanity