is body in the first instant of its conception.

Obj. 2: Further, the soul, like any natural form, requires
determinate quantity in its matter. But in the first instant of its
conception Christ's body was not of the same quantity as the bodies
of other men when they are animated: otherwise, if afterwards its
development had been continuous, either its birth would have occurred
sooner, or at the time of birth He would have been a bigger child
than others. The former alternative is contrary to what Augustine
says (De Trin. iv), where he proves that Christ was in the Virgin's
womb for the space of nine months: while the latter is contrary to
what Pope Leo says (Serm. iv in Epiph.): "They found the child Jesus
nowise differing from the generality of infants." Therefore Christ's
body was not animated in the first instant of its conception.

Obj. 3: Further, whenever there is "before" and "after" there must be
several instants. But according to the Philosopher (De Gener. Animal.
ii) in the generation of a man there must needs be "before" and
"after": for he is first of all a living thing, and afterwards, an
animal, and after that, a man. Therefore the animation of Christ
could not be effected in the first instant of His conception.

_On the contrary,_ Damascene says (De Fide Orth. iii): "At the very
instant that there was flesh, it was the flesh of the Word of God, it
was flesh animated with a rational and intellectual soul."

_I answer that,_ For the conception to be attributed to the very Son
of God, as we confess in the Creed, when we say, "who was conceived
by the Holy Ghost," we must needs say that the body itself, in being
conceived, was assumed by the Word of God. Now it has been shown
above (Q. 6, AA. 1, 2) that the Word of God assumed the body by means
of the soul, and the soul by means of the spirit, i.e. the intellect.
Wherefore in the first instant of its conception Christ's body must
needs have been animated by the rational soul.

Reply Obj. 1: The beginning of the infusion of the soul may be
considered in two ways. First, in regard to the disposition of the
body. And thus, the beginning of the infusion of the soul into
Christ's body was the same as in other men's bodies: for just as the
soul is infused into another man's body as soon as it is formed, so
was it with Christ. Secondly, this beginning may be considered merely
in regard to time. And thus, because Christ's body was perfectly
formed in a shorter s