obverse

2. The side of something that is meant to be seen or is most important.

The obverse of the argument is that costs will rise.

3. A counterpart or opposite that corresponds to something else.

Every problem has its obverse, a chance to learn.

4. In logic, a proposition formed by replacing a term with its negation and changing the quality.

The obverse of 'all dogs are friendly' is 'no dogs are unfriendly.'