Qiyas

The method of deriving a ruling for a new situation by analogy to a case addressed in the Qur'an or Sunnah, when both share the same underlying cause (illah). It is the fourth source of Sunni law and the main tool by which classical rulings reach modern finance: paper money is treated like gold and silver because it serves as money; corporations are analyzed through partnership rules; and derivatives are judged by analogy to classical sales with excessive uncertainty.

Scholars disagree about the correct illah in many cases, which produces different modern rulings from the same texts.

Almost every modern Islamic finance ruling is a qiyas, which is why the same hadith can yield different verdicts from different scholars. The clearest example is money itself. The hadith on riba names gold and silver.

Is a dollar like gold because it is a medium of exchange (so all currencies are ribawi and must be exchanged on the spot), or is it unlike gold because it has no intrinsic value (so some rules relax)? Nearly all contemporary scholars take the first view, which is why forex rules, Sarf, and the prohibition on discounting debts apply to dollars. A second example is the corporation: by analogy to classical partnership, a shareholder is a part-owner, so buying a stock is buying a share of the company's assets and activities, which is why the company's business and balance sheet are screened; scholars who weight the analogy differently reach different debt thresholds. A third is cryptocurrency, where the analogy chosen (currency, commodity, or digital asset with no counterpart) drives the ruling.

For consumers the lesson is that disagreements among qualified scholars are usually disagreements about which analogy fits, not about the texts, and that a fatwa worth following states its reasoning. When you read a provider's Shariah certificate or a verdict on a stock or coin, look for the analogy: a ruling that names the classical case and the shared cause can be evaluated; one that simply pronounces cannot.