Objection 1. It would seem that Christ did not suffer in His whole soul. For the soul suffers indirectly when the body suffers, inasmuch as it is the "act of the body." But the soul is not, as to its every part, the "act of the body"; because the intellect is the act of no body, as is said De Anima iii. Therefore it seems that Christ did not suffer in His whole soul.
Objection 2. Further, every power of the soul is passive in regard to its proper object. But the higher part of reason has for its object the eternal types, "to the consideration and consultation of which it directs itself," as Augustine says (De Trin. xii). But Christ could suffer no hurt from the eternal types, since they are nowise opposed to Him. Therefore it seems that He did not suffer in His whole soul.
Objection 3. Further, a sensitive passion is said to be complete when it comes into contact with the reason. But there was none such in Christ, but only "pro-passions"; as Jerome remarks on Matthew 26:37. Hence Dionysius says in a letter to John the Evangelist that "He endured only mentally the sufferings inflicted upon Him." Consequently it does not seem that Christ suffered in His whole soul.
Objection 4. Further, suffering causes pain: but there is no pain in the speculative intellect, because, as the Philosopher says (Topic. i), "there is no sadness in opposition to the pleasure which comes of consideration." Therefore it seems that Christ did not suffer in His whole soul.
On the contrary, It is written (Psalm 87:4) on behalf of Christ: "My soul is filled with evils": upon which the gloss adds: "Not with vices, but with woes, whereby the soul suffers with the flesh; or with evils, viz. of a perishing people, by compassionating them." But His soul would not have been filled with these evils except He had suffered in His whole soul. Therefore Christ suffered in His entire 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.
Reply to Objection 1. Although the intellect as a faculty is not the act of the body, still the soul's essence is the act of the body, and in it the intellective faculty is rooted, as was shown in I, 77, 6,8.
Reply to Objection 2. This argument proceeds from passion on the part of the proper object, according to which Christ's higher reason did not suffer.
Reply to Objection 3. Grief is then said to be a true passion, by which the soul is troubled, when the passion in the sensitive part causes reason to deflect from the rectitude of its act, so that it then follows the passion, and has no longer free-will with regard to it. In this way passion of the sensitive part did not extend to reason in Christ, but merely subjectively, as was stated above.
Reply to Objection 4. The speculative intellect can have no pain or sadness on the part of its object, which is truth considered absolutely, and which is its perfection: nevertheless, both grief and its cause can reach it in the way mentioned above.
Source: Summa Theologica (New Advent)