We believe that all the prophets are infallible, and also that the Imams (A.S) (the successors to the prophet of Islam) are infallible, but some non-Shi'a Muslim do not believe in the infallibility of the prophets, let alone of the Imams. Infallibility means purity from all sins, both major and minor ones,and from mistakes and forgetfulness. It is necessary that a prophet should not even do what is contrary to good manners: that is, by laughing aloud, or by doing anything which may be unacceptable to public opinion.
The reason for the necessity of the infallibility of a prophet is that if he commits a sin or mistakes, or is forgetful or something similar, we have to choose between two alternatives: either we obey his sins and mistakes, which too is wrong, or we must not obey his idea of prophethood where obedience is necessary; besides, if everything he says or does has the possibility of being either right or wrong, then it is impossible for us to follow him. The result is that the benefit of his mission is lost; it becomes unnecessary , and the prophet become like ordinary people whose acts and speech do not have the excellent worth that we seek, with the result that there will be no obedience and his actions will be unreliable.
The same reason is adduced for the infallibility of the Imams (A.S), because we believe that the Imam is appointed by Allah as the Prophet's representative (khalifah) to guide mankind. This will be explained in the section on the Imamate.
