What is Qiyas (analogical reasoning) in Sharia, what are its pillars, and what is its legal probative force?

Linguistically, Qiyas (analogy) means estimation and equalization. Technically, it is a method of inference by which the mujtahid (independent jurist) attaches an issue unspoken about in the Sharia to the ruling of a textually stated matter, if they share the same effective cause for the ruling. Its pillars are four: the original (the subject of comparison, or whose ruling is known), the ruling of the original, the 'illa (effective cause - the common meaning between the original and the derived issue), and the far' (derived issue - the thing for which no textual ruling has been revealed). Qiyas is considered a valid proof by the majority of scholars because the texts of revelation are limited while incidents are ever-renewing. It is the fourth principle of deduction, following the Quran, Sunnah, and Ijma' (consensus).