/summa/SS/SSII/SS_Q110:
We must now consider the vices opposed to truth, and (1) lying: (2) dissimulation or hypocrisy: (3) boasting and the opposite vice. Concerning lying there are four points of inquiry:
(1) Whether lying, as containing falsehood, is always opposed to truth?
(2) Of the species of lying;
(3) Whether lying is always a sin?
(4) Whether it is always a mortal sin?