Objection 1: It would seem that the Holy Ghost is not fittingly sent in a visible manner. For the Son as visibly sent to the world is said to be less than the Father. But the Holy Ghost is never said to be less than the Father. Therefore the Holy Ghost is not fittingly sent in a visible manner.
Obj. 2: Further, the visible mission takes place by way of union to a visible creature, as the Son's mission according to the flesh. But the Holy Ghost did not assume any visible creature; and hence it cannot be said that He exists otherwise in some creatures than in others, unless perhaps as in a sign, as He is also present in the sacraments, and in all the figures of the law. Thus the Holy Ghost is either not sent visibly at all, or His visible mission takes place in all these things.
Obj. 3: Further, every visible creature is an effect showing forth the whole Trinity. Therefore the Holy Ghost is not sent by reason of those visible creatures more than any other person.
Obj. 4: Further, the Son was visibly sent by reason of the noblest kind of creature—namely, the human nature. Therefore if the Holy Ghost is sent visibly, He ought to be sent by reason of rational creatures.
Obj. 5: Further, whatever is done visibly by God is dispensed by the ministry of the angels; as Augustine says (De Trin. iii, 4,5,9). So visible appearances, if there have been any, came by means of the angels. Thus the angels are sent, and not the Holy Ghost.
Obj. 6: Further, the Holy Ghost being sent in a visible manner is only for the purpose of manifesting the invisible mission; as invisible things are made known by the visible. So those to whom the invisible mission was not sent, ought not to receive the visible mission; and to all who received the invisible mission, whether in the New or in the Old Testament, the visible mission ought likewise to be sent; and this is clearly false. Therefore the Holy Ghost is not sent visibly.
On the contrary, It is said (Matt. 3:16) that, when our Lord was baptized, the Holy Ghost descended upon Him in the shape of a dove.
I answer that, God provides for all things according to the nature of each thing. Now the nature of man requires that he be led to the invisible by visible things, as explained above (Q. 12, A. 12). Wherefore the invisible things of God must be made manifest to man by the things that are visible. As God, therefore, in a certain way has demonstrated Himself and His eternal processions to men by visible creatures, according to certain signs; so was it fitting that the invisible missions also of the divine persons should be made manifest by some visible creatures.
This mode of manifestation applies in different ways to the Son and to the Holy Ghost. For it belongs to the Holy Ghost, Who proceeds as Love, to be the gift of sanctification; to the Son as the principle of the Holy Ghost, it belongs to the author of this sanctification. Thus the Son has been sent visibly as the author of sanctification; the Holy Ghost as the sign of sanctification.
Reply Obj. 1: The Son assumed the visible creature, wherein He appeared, into the unity of His person, so that whatever can be said of that creature can be said of the Son of God; and so, by reason of the nature assumed, the Son is called less than the Father. But the Holy Ghost did not assume the visible creature, in which He appeared, into the unity of His person; so that what is said of it cannot be predicated of Him. Hence He cannot be called less than the Father by reason of any visible creature.