The paired language of ṭard and ʿaks

Analogy: Original and New Cases, Common Cause, Co-variation and Elimination

الدوران عند المتأخرين، والقدماء كانوا يسمونها بالطرد والعكس
Later scholars call this dawārān, whereas earlier scholars referred to it as ṭard and ʿaks.

Here ṭard is the positive accompaniment of a feature and its judgment, while ʿaks is their corresponding absence. Together they make up the two sides of co-variation.

This use of ʿaks is distinct from conversion of propositions (ʿaks mustawī and ʿaks naqīḍ) studied previously.