1.
A.is the one case of conversion by limitation: All S is P; .'.

Some P is S.
All cats are grey in the dark; .'.

Some things grey in the dark are cats.
The predicate is treated as particular, when taking it for the new subject, according to the rule not to go beyond the evidence.

To infer that _All things grey in the dark are cats_ would be palpably absurd; yet no error of reasoning is commoner than the simple conversion of A.
The validity of conversion by limitation may be shown thus: if, _All S is P_, then, by subalternation, _Some S is P_, and therefore, by simple conversion, _Some P is S_.
O.cannot be truly converted.

If we take the proposition: _Some S is not P_, to convert this into _No P is S_, or _Some P is not S_, would break the rule in chap.vi.Sec.