(1) “All earnest teachers are diligent students.”
(2) “No self-respecting man can afford to be careless in his personal appearance.”
(3) “Some of the great teachers of the past did not practice what they preached.”
(4) “Some weeds are beautiful.”
I determine first the logical character of each proposition, finding the first to be an A, the second an E, the third an O and the fourth an I. Then I recall that in obversion the predicate must always be negated and an A must be changed to an E or an E to an A; also an I must be changed to an O or an O to an I. Hence, the obverse of each proposition is:
(1) “No earnest teacher is a not-diligent student.”
(2) “All self-respecting men can afford to be not-careless (careful) in their personal appearance.”
(3) “Some of the great teachers of the past did not-practice (failed to practice) what they preached.”
(4) “Some weeds are not not-beautiful.”
(4b) Infer by obversion from the following: