Obversion of Categorical Statements in Logic

In logic, it's possible to convert, obvert or contrapose categorical statements. Obversion is done by changing the quality rather than the quantity of a categorical statement and by replacing the predicate term with its complement (i.e., everything that does not belong to the class of the predicate term).

Example:

Statement 1) "All Athenians are Greeks."

The obversion of statement 1) is "No Athenians are non-Greeks."

In this case, we have obverted a universal affirmative (A statement) by changing its quality to a negative and "Greeks" (the predicate term) has been replaced with its complement term ("non-Greeks").

A statements are universal affirmatives
E statements are universal negatives
I statements are particular affirmatives
O statements are particular negatives (i.e., "some S are not P" or "some S is not P")