/summa/SS/SSII/SS_Q70:
We must now consider injustice with regard to the person of the witness. Under this head there are four points of inquiry:
(1) Whether a man is bound to give evidence?
(2) Whether the evidence of two or three witnesses suffices?
(3) Whether a man's evidence may be rejected without any fault on his part?
(4) Whether it is a mortal sin to bear false witness?