إذا قال الحرّة: «بعت نفسي منك» صار مجازاً عن النكاح
In a marriage setting, a free woman saying 'I have sold myself to you' is discussed as figuratively indicating marriage.
Because a free person cannot be literally sold as property, the ordinary sale meaning has no valid subject. The unsuitability of the literal object supplies a reason to consider a recognized alternative legal meaning.
«هذا ابني» مع ثبوت النسب من غيره
Likewise, 'This is my son' cannot establish literal fatherhood where the relevant parentage is already established elsewhere.
These examples concern distinct legal subjects—marriage and emancipation. Their common interpretive structure is that the proposed literal effect cannot attach to the subject of the speech.