«ويعرف العلة بالكتاب والسنة والإجماع والاجتهاد»
The effective cause is known through the Qur’an, Sunnah, ijma‘, and ijtihad.
Qur’an: textual wording or implication indicates a cause.
Sunnah: a prophetic report indicates an operative feature.
Ijma‘: established consensus identifies the role of a feature.
Ijtihad: qualified juristic reasoning derives a cause where it is not expressly stated.
These are not licenses to choose a convenient explanation. Evidence and sound legal method are required, and the textbook supplies a distinct illustration of each path.