/summa/SS/SSII/SS_Q103:
We must now consider the parts of observance. We shall consider (1) dulia, whereby we pay honor and other things pertaining thereto to those who are in a higher position; (2) obedience, whereby we obey their commands.
Under the first head there are four points of inquiry:
(1) Whether honor is a spiritual or a corporal thing?
(2) Whether honor is due to those only who are in a higher position?
(3) Whether dulia, which pays honor and worship to those who are above us, is a special virtue, distinct from latria?
(4) Whether it contains several species?