/summa/TP/TPII/TP_Q32:
We shall now consider the active principle in Christ's conception: concerning which there are four points of inquiry:
(1) Whether the Holy Ghost was the active principle of Christ's conception?
(2) Whether it can be said that Christ was conceived of the Holy Ghost?
(3) Whether it can be said that the Holy Ghost is Christ's father according to the flesh?
(4) Whether the Blessed Virgin cooperated actively in Christ's conception?