The Hanafi and Shafi'i madhab (schools of jurisprudence) define gharar as "that whose consequences are hidden,"[20] the Hanbali school as "that whose consequences are unknown" or "that which is undeliverable, whether it exists or not." One modern scholar of Islam, Mustafa Al-Zarqa, defines gharar as "the sale of probable items whose existence or characteristics are not certain, due to the risky nature that makes the trade similar to gambling."[18