Objection 1. It would seem that outward pain is greater than interior sorrow of the heart. Because outward pain arises from a cause repugnant to the well-being of the body in which is life: whereas interior sorrow is caused by some evil in the imagination. Since, therefore, life is loved more than an imagined good, it seems that, according to what has been said above (Article 6), outward pain is greater than interior sorrow.
Objection 2. Further, the reality moves more than its likeness does. But outward pain arises from the real conjunction of some contrary; whereas inward sorrow arises from the apprehended likeness of a contrary. Therefore outward pain is greater than inward sorrow.
Objection 3. Further, a cause is known by its effect. But outward pain has more striking effects: since man dies sooner of outward pain than of interior sorrow. Therefore outward pain is greater and is shunned more than interior sorrow.
On the contrary, it is written (Sirach 25:17): "The sadness of the heart is every wound [Douay: 'plague'], and the wickedness of a woman is all evil." Therefore, just as the wickedness of a woman surpasses all other wickedness, as the text implies; so sadness of the heart surpasses every outward wound.
I answer that, Interior and exterior pain agree in one point and differ in two. They agree in this, that each is a movement of the appetitive power, as stated above (Article 1). But they differ in respect of those two things which are requisite for pain and pleasure; namely, in respect of the cause, which is a conjoined good or evil; and in respect of the apprehension. For the cause of outward pain is a conjoined evil repugnant to the body; while the cause of inward pain is a conjoined evil repugnant to the appetite. Again, outward pain arises from an apprehension of sense, chiefly of touch; while inward pain arises from an interior apprehension, of the imagination or of the reason.
If then we compare the cause of inward pain to the cause of outward pain, the former belongs, of itself, to the appetite to which both these pains belong: while the latter belongs to the appetite directly. Because inward pain arises from something being repugnant to the appetite itself, while outward pain arises from something being repugnant to the appetite, through being repugnant to the body. Now, that which is of itself is always prior to that which is by reason of another. Wherefore, from this point of view, inward pain surpasses outward pain. In like manner also on the part of apprehension: because the apprehension of reason and imagination is of a higher order than the apprehension of the sense of touch. Consequently inward pain is, simply and of itself, more keen than outward pain: a sign whereof is that one willingly undergoes outward pain in order to avoid inward pain: and in so far as outward pain is not repugnant to the interior appetite, it becomes in a manner pleasant and agreeable by way of inward joy. Sometimes, however, outward pain is accompanied by inward pain, and then the pain is increased. Because inward pain is not only greater than outward pain, it is also more universal: since whatever is repugnant to the body, can be repugnant to the interior appetite; and whatever is apprehended by sense may be apprehended by imagination and reason, but not conversely. Hence in the passage quoted above it is said expressively: "Sadness of the heart is every wound," because even the pains of outward wounds are comprised in the interior sorrows of the heart.
Reply to Objection 1. Inward pain can also arise from things that are destructive of life. And then the comparison of inward to outward pain must not be taken in reference to the various evils that cause pain; but in regard to the various ways in which this cause of pain is compared to the appetite.
Reply to Objection 2. Inward pain is not caused by the apprehended likeness of a thing: for a man is not inwardly pained by the apprehended likeness itself, but by the thing which the likeness represents. And this thing is all the more perfectly apprehended by means of its likeness, as this likeness is more immaterial and abstract. Consequently inward pain is, of itself, greater, as being caused by a greater evil, forasmuch as evil is better known by an inward apprehension.
Reply to Objection 3. Bodily changes are more liable to be caused by outward pain, both from the fact that outward pain is caused by a corruptive conjoined corporally, which is a necessary condition of the sense of touch; and from the fact that the outward sense is more material than the inward sense, just as the sensitive appetite is more material than the intellective. For this reason, as stated above (22, 3; 31, 5), the body undergoes a greater change from the movement of the sensitive appetite: and, in like manner, from outward than from inward pain.
Source: Summa Theologica (New Advent)