is somewhat more intricate. It may be indicated by DOcsAmOsc. You substitute for the Major Premiss its Converse by Contraposition, transpose the Premisses and you have DArII.
All M is in S.
Some not-P is in M.
Some not-P is in S.
Convert now the conclusion by Contraposition, and you have Some S is not in P.
The author of the Mnemonic apparently did not recognise Contraposition, though it was admitted by Boethius; and, it being impossible without this to demonstrate the validity of Baroko and Bokardo by showing them to be equivalent with valid moods of the First Figure, he provided for their demonstration by the special process known as Reductio ad absurdum. B indicates that Barbara is the medium.
The rationale of the process is this. It is an imaginary opponent that you reduce to an absurdity or self-contradiction. You show that it is impossible with consistency to admit the premisses and at the same time deny the conclusion. For, let this be done; let it be admitted as in BArOkO that,
All P is in M
Some S is not in M,