I answer that, As stated above (A. 1), spiritual perfection was becoming to the human nature which Christ took, which perfection He attained not by making progress, but by receiving it from the very first. Now ultimate perfection does not consist in power or habit, but in operation; wherefore it is said (De Anima ii, text. 5) that operation is a "second act." We must, therefore, say that in the first instant of His conception Christ had that operation of the soul which can be had in an instant. And such is the operation of the will and intellect, in which the use of free-will consists. For the operation of the intellect and will is sudden and instantaneous, much more, indeed, than corporeal vision; inasmuch as to understand, to will, and to feel, are not movements that may be described as "acts of an imperfect being," which attains perfection successively, but are "the acts of an already perfect being," as is said, De Anima iii, text. 28. We must therefore say that Christ had the use of free-will in the first instant of His conception.
Reply Obj. 1: Existence precedes action by nature, but not in time; but at the same time the agent has perfect existence, and begins to act unless it is hindered. Thus fire, as soon as it is generated, begins to give heat and light. The action of heating, however, is not terminated in an instant, but continues for a time; whereas the action of giving light is perfected in an instant. And such an operation is the use of free-will, as stated above.
Reply Obj. 2: As soon as counsel or deliberation is ended, there may be choice. But those who need the deliberation of counsel, as soon as this comes to an end are certain of what ought to be chosen: and consequently they choose at once. From this it is clear that the deliberation of counsel does not of necessity precede choice save for the purpose of inquiring into what is uncertain. But Christ, in the first instant of His conception, had the fulness of sanctifying grace, and in like manner the fulness of known truth; according to John 1:14: "Full of grace and truth." Wherefore, as being possessed of certainty about all things, He could choose at once in an instant.
Reply Obj. 3: Christ's intellect, in regard to His infused knowledge, could understand without turning to phantasms, as stated above (Q. 11, A. 2). Consequently His intellect and will could act without any action of the senses.
Nevertheless it was possible for Him, in the first instant of His conception, to have an operation of the senses: especially as to the sense of touch, which the infant can exercise in the womb even before it has received the rational soul, as is said, De Gener. Animal. ii, 3, 4. Wherefore, since Christ had the rational soul in the first instant of His conception, through His body being already fashioned and endowed with sensible organs, much more was it possible for Him to exercise the sense of touch in that same instant. _______________________
THIRD ARTICLE [III, Q. 34, Art. 3]
Whether Christ Could Merit in the First Instant of His Conception?
Objection 1: It would seem that Christ could not merit in the first instant of His conception. For the free-will bears the same relation to merit as to demerit. But the devil could not sin in the first instant of his creation, as was shown in the First Part, Q. 63, A. 5. Therefore neither could Christ's soul merit in the first instant of its creation—that is, in the first instant of Christ's conception.
Obj. 2: Further, that which man has in the first instant of his conception seems to be natural to him: for it is in this that his natural generation is terminated. But we do not merit by what is natural to us, as is clear from what has been said in the Second Part (I-II, Q. 109, A. 5; Q. 114, A. 2). Therefore it seems that the use of free-will, which Christ as man had in the first instant of His conception, was not meritorious.
Obj. 3: Further, that which a man has once merited he makes, in a way, his own: consequently it seems that he cannot merit the same thing again: for no one merits what is already his. If, therefore, Christ merited in the first instant of His conception, it follows that afterwards He merited nothing. But this is evidently untrue. Therefore Christ did not merit in the first instant of His conception.