(A per acc.) If a thing is common, it is always cheap.
.'. If a thing is cheap, it is sometimes not uncommon. (O)
§ 723. When applied to disjunctive propositions, the distinctive features of conversion by negation are still discernible. In each of the following forms of inference the converse differs in quality from the convertend and has the contradictory of one of the original terms (§ 515).
§ 724.
(A) Either A is B or C is always D.
.'. Either C is D or A is never not-B. (E)
(E) Either A is B or C is never D.
.'. Either C is not-D or A is always B. (A)
(I) Either A is B or C is sometimes D.
.'. Either C is not-D or A is sometimes not B. (O)
(O) Either A is B or C is sometimes not D.
.'. Either C is D or A is sometimes not-B. (I)
§ 725.
(A) Either miracles are possible or every ancient historian is
untrustworthy.
.'. Either ancient historians are untrustworthy or miracles are
not impossible. (E)
(E) Either the tide must turn or the vessel can not make the port.
.'. Either the vessel cannot make the port or the tide must
turn. (A)