«يَنْزِلُهُمَا مَنْزِلَةَ التَّضَايُفِ»
It treats the two as though they stood in a correlative relation.
The intellect may find opposition or incompatibility, yet estimation quickly recalls one opposite upon encountering the other. In this respect the pair behaves as if the members called one another to mind.
This is a likening to correlation, not the claim that every contrary is literally a relative such as 'father' and 'son.' The issue is mental association, not a change in the external essences of things.