Objection 1. It would seem that the causes of divorce had to be written in the bill: because the husband was absolved from the punishment of the law by the written bill of divorce. But this would seem altogether unjust, unless sufficient causes were alleged for a divorce. Therefore it was necessary for them to be written in the bill.
Objection 2. Further, seemingly this document was of no use except to show the causes for divorce. Therefore, if they were not written down, the bill was delivered for no purpose.
Objection 3. Further, the Master says that it was so in the text (Sent. iv, D, 33).
On the contrary, The causes for divorce were either sufficient or not. If they were sufficient, the wife was debarred from a second marriage, though this was allowed her by the Law. If they were insufficient, the divorce was proved to be unjust, and therefore could not be effected. Therefore the causes for divorce were by no means particularized in the bill.
I answer that, The causes for divorce were not particularized in the bill, but were indicated in a general way, so as to prove the justice of the divorce. According to Josephus (Antiq. iv, 6) this was in order that the woman, having the written bill of divorce, might take another husband, else she would not have been believed. Wherefore according to him it was written in this wise: "I promise never to have thee with me again." But according to Augustine (Contra Faust. xix, 26) the bill was put into writing in order to cause a delay, and that the husband might be dissuaded by the counsel of the notaries to refrain from his purpose of divorce.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)