«مَنْ يَعْتَقِدُ الْعَكْسَ»
A listener who believes the opposite of what the speaker asserts.
Qalb (قصر قلب) corrects a reversed assignment. Imagine the listener believes that a person is a poet rather than a writer, while the speaker establishes the reverse: he is a writer, not a poet. The utterance does more than add information: it overturns an existing contrary judgment.
In ifrād the listener believes “both”; in qalb the listener believes the wrong one. Hence similar wording can function as ifrād or qalb depending on the listener’s prior view.