/summa/FS/FSIV/FS_Q51:
We must next consider the cause of habits: and firstly, as to their formation; secondly, as to their increase; thirdly, as to their diminution and corruption. Under the first head there are four points of inquiry:
(1) Whether any habit is from nature?
(2) Whether any habit is caused by acts?
(3) Whether any habit can be caused by one act?
(4) Whether any habits are infused in man by God?