Hawzah News Agency (Qom, Iran) – In addition to being incapable of sin, the Messengers are also immune from error in the following domains:
1. Judging disputes. The Messengers were charged by God to pass judgment according to the divinely-instituted scales of justice, and there could never be any question of the Messengers deviating in any way from the principles by which disputes and all other legal questions were to be adjudicated.
2. Specification of the boundaries of religious rulings: for example, ascertaining whether a given liquid is alcoholic or not.
3. The domain of social principles: for example, specifying what contributes to public welfare and what corrupts it.
4. The domain of conventional daily matters.
The reason for the ‘isma operating in the last three domains is this: In the minds of most people, error in these matters implies error in the domain of religious rulings also. Consequently, committing error in such matters undermines the certainty that people must have in the personality of the Messenger, and leads ultimately to the undermining of the purpose of prophecy. However, the necessity of ‘isma in the first two domains is more readily apparent than in the fourth.
Ayatollah Jafar Sobhani, Doctrines of Shii Islam, A Compendium of Imami Beliefs and Practices, Translated and Edited by Reza Shah-Kazemi, published by I.B.Tauris Publishers, London • New York 2003.