The book identifies four principal sources: Qur'an, Sunnah, ijma and qiyas. The first two provide transmitted texts; ijma concerns qualified juristic consensus; qiyas extends a ruling to a case without a specific text through a shared effective cause.
These are not interchangeable labels. Widespread agreement is not automatically ijma, and a superficial resemblance between cases is not sufficient for qiyas. Each source has its own requirements and evidentiary standing.