Some philosophers are not inconsistent._ In agreement with this mode of inference, we have the rule of modern English grammar, that 'two negatives make an affirmative.' Again, by the principle of Excluded Middle, if any term be denied of a subject, its contradictory may be affirmed: to obvert negative propositions, then, the rule is--Remove the negative sign, and for the predicate substitute its contradictory term.
E._No S is P .'.

All S is not-P No matter is destructible .'.

All matter is indestructible._ O._Some S is not P .'.

Some S is not-P Some ideals are not attainable .'.

Some ideals are unattainable._ Thus, by obversion, each of the four propositions retains its quantity but changes its quality: A.to E., I.to O., E.to A., O.to I.And all the obverses are infinite propositions, the affirmative infinites having the sense of negatives, and the negative infinites having the sense of affirmatives.
Again, having obtained the obverse of a given proposition, it may be desirable to recover the obvertend; or it may at any time be requisite to change a given infinite proposition into the corresponding direct affirmative or negative; and in such cases the process is still obversion.