Page "Deontic logic" Paragraph 34

From these axioms Mally deduced 35 theorems, many of which he rightly considered strange.

Karl Menger showed that! A ↔ A is a theorem and thus that the introduction of the!

sign is irrelevant and that A ought to be the case if A is the case.

After Menger, philosophers no longer considered Mally's system viable.

Gert Lokhorst lists Mally's 35 theorems and gives a proof for Menger's theorem at the Stanford Encyclopedia of Philosophy under Mally's Deontic Logic.