والافتراق: وهو كونهما بحيث يمكن أن يتوسطهما ثالث
Separation is the relation of two substances such that a third could be situated between them.
Unlike conjunction, the condition is the possibility of an intervening third, not its actual presence. Possibility and actuality must be kept distinct.
Imagine a gap between two substances: technical separation does not require a third item already occupying it. It requires only that the relation permits such an intervening occupant.