Analogical Reasoning (qiyās)

Definition
Linguistically, qiyās means estimation and equivalence. Technically, it is to treat a subsidiary case in the same way as an original case with respect to a ruling because the two share a unifying effective cause.

The subsidiary case (farʿ) is the matter being compared.
The original case (aṣl) is the precedent to which it is compared.
The ruling is whatever the revealed evidence requires—obligation, prohibition, validity, invalidity, and so forth.
The effective cause (ʿillah) is the rationale on account of which the ruling was established in the original case.

These four are the pillars of qiyās, and qiyās is one of the proofs by which legal rulings are derived.

Its legitimacy as a legal proof is attested by the Qur’an, the Sunnah, and the statements of the Companions. From the Qur’an:

1 – “Allah is the One who sent down the Book in truth and the Balance” ⦗al-Shūrā 42:17⦘. The Balance is what things are weighed with and by which they are mutually compared.

2 – “Just as We began the first creation, We shall recreate it” ⦗al-Anbiyāʾ 21:104⦘, and: “Allah is the One who sends the winds; they stir up clouds, then We drive them to a lifeless land and revive the earth after its death—such is the Resurrection” ⦗Fāṭir 35:9⦘.

Here Allah likens the second creation to the first, and likens the revivification of the dead to the reviving of the earth; that is the very essence of qiyās.