Question Twelve of Chapter Nine of the Third Decade
Whether it is lawful to use red-hot iron or boiling water for the investigation of something hidden
It is asked whether it is lawful to arrange a judgment of red-hot iron or boiling water for the investigation of someone’s hidden sin.
It is answered that it is not, and this for three reasons.
The first is that it is ordained for judging hidden matters which are reserved for divine judgment.
The second reason is that in such practices a miracle is expected, which is not free from tempting God.
The third reason is that it is condemned by canon law.
Hence, in cause 2, question 5, chapter Consuluisti, in a decree of Pope Stephen V, it is said: “The sacred canons do not judge that an examination by red-hot iron or boiling water and a confession should be extorted from anyone; and what has not been sanctioned by the teaching of the holy fathers ought not to be presumed by a superstitious invention. For offenses made public through a voluntary confession or through the approval of witnesses have been committed to our governance for judgment, with the fear of God held before our eyes. Hidden and unknown matters, however, must be left to him who alone knows the hearts of the children of human beings.”
For the same reasons, the judgment of duels also appears to be prohibited, as Thomas says in the Second Part of the Second Part, question 95, article 8.
Addition
It must be noted that Nicholas de Lyra, upon 1 Kings 17, on the occasion of the duel or conflict of David with the Philistine, draws the conclusion that in some case a duel can be lawful. He asserts that this is according to the mind of Saint Thomas, which is nevertheless false; therefore he is rightly reproved by Paul of Burgos, a most learned man.
This holds unless we wish to say that, when it is said that a duel can occur in some case without mortal sin, the statement is made from the position of those who judge or give counsel, where such an ordeal is freely carried out not through their incitement or counsel, but as regards the one issuing and the one accepting the challenge.
But let everyone—and especially princes—know that the use of duels is altogether prohibited by a decree of the sacred Council of Trent, Session 25, chapter 19.