Do not assume that every idafah is definite. If the mudaf ilayh is indefinite, the phrase remains indefinite, although its meaning becomes more specific.
كِتَابُ وَلَدٍ means “a boy’s book.” We know what kind of book is meant, but the boy is not identified.
So the phrase gains specification (تخصیص), not definiteness.
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Do not assume that every idafah is definite. If the mudaf ilayh is indefinite, the phrase remains indefinite, although its meaning becomes more specific.
كِتَابُ وَلَدٍ means “a boy’s book.” We know what kind of book is meant, but the boy is not identified.
So the phrase gains specification (تخصیص), not definiteness.