Summa Theologica, Part III, Question 46. The passion of Christ
Article 7. Whether Christ suffered in His whole soul?
I answer that, A whole is so termed with respect to its parts. But the parts of a soul are its faculties. So, then, the whole soul is said to suffer in so far as it is afflicted as to its essence, or as to all its faculties. But it must be borne in mind that a faculty of the soul can suffer in two ways: first of all, by its own passion; and this comes of its being afflicted by its proper object; thus, sight may suffer from superabundance of the visible object. In another way a faculty suffers by a passion in the subject on which it is based; as sight suffers when the sense of touch in the eye is affected, upon which the sense of sight rests, as, for instance, when the eye is pricked, or is disaffected by heat.
So, then, we say that if the soul be considered with respect to its essence, it is evident that Christ’s whole soul suffered. For the soul’s whole essence is allied with the body, so that it is entire in the whole body and in its every part. Consequently, when the body suffered and was disposed to separate from the soul, the entire soul suffered. But if we consider the whole soul according to its faculties, speaking thus of the proper passions of the faculties, He suffered indeed as to all His lower powers; because in all the soul’s lower powers, whose operations are but temporal, there was something to be found which was a source of woe to Christ, as is evident from what was said above (Article 6). But Christ’s higher reason did not suffer thereby on the part of its object, which is God, who was the cause, not of grief, but rather of delight and joy, to the soul of Christ. Nevertheless, all the powers of Christ’s soul did suffer according as any faculty is said to be affected as regards its subject, because all the faculties of Christ’s soul were rooted in its essence, to which suffering extended when the body, whose act it is, suffered.