«حفظ النسبة ليس بضروري في العكس»
Maintaining every relational qualification unchanged is not necessary in conversion.
The book illustrates the issue through youth and old age: a young person may become old at another time, which does not imply simultaneous youth and old age.
The primary requirements are preservation of truth and quality. Yet when time or another qualification affects the meaning, the valid converse must be interpreted carefully. A simple word reversal does not settle the temporal scope.