Can you explain the idea of Infallibility according to Islam? Are Prophets and Imams literally incapable of sinning?
Etymology of al-`Esma:
Al-`Esma is the Arabic root of the term under discussion which literally means taking hold of something; abstaining.
In the story of Prophet Noah, with reference to this literal meaning he proclaims to his son: “There is no protector today from the punishment of God” (Hud:43). We know that whenever there is a protector there will naturally be a protected one as well.
Al-`Esma also means a bracelet. Thus, wrist in Arabic is called; Al-me`sam.
By definition, the term fallible means capable of making an error. As such, the term infallible (in = not) means incapable of error or failing. By contrast, al-‘esma (of the Prophets) means that God has protected them by virtue of their pure soul, and by helping them with resistance, tranquility and blessing . Thus making them ma`soum. In reference to the Prophet of Islam, the Qur`an says: ‘God will protect you from people’.
Due to the nature of their creation, angels are incapable of committing sin, because they lack the human desires and the power of choice. This act can best be described by using the following analogies. A human who is born blind is incapable of having a lustful look simply because he lacks the ability (in this case, eyesight) to do so. Being incapable of committing this sin makes the blind person infallible in this regard, but not ma`soum, as they have no discretion or choice in the matter. Similarly, an infant is incapable of committing adultery because his potency is not yet developed. Incapacity in this sense is not a virtue. An infant is not worthy of praise for its chastity. We only admire the one who, although was capable of indulging in the sin, was able to control and protect himself against it.
Prophets, like any other human, by virtue enjoy the blessing of freewill, and hence are potentially capable of committing a sin or erring. By applying the term al-‘esma to the Prophet, we do not mean that they are infallible or incapable of making a mistake (as in the case of angels). Rather, they are capable of indulging in the sin, but are able to control and protect themselves against it. This reasoning is part of the solution to the argument posed by our Christian brethren, when they ask ‘how can a human be infallible?` Despite the fact that, ironically, Catholics believe in the infallibility of the Pope.
Nonetheless, as the term ‘Infallibility` is widely used, I shall also provisionally use the term for the translation of ‘al-‘esma` until I suggest a more accurate translation.
As stated, by definition, ‘Infallibility` in Islam does not mean incapability of committing sin or erring (such as in the realm of angels). Infallibility in this sense is in fact not a virtue, and is in conflict with the human nature of free will. Hosham, the student of Imam Sadiq (a.s) in theology says: I asked Imam Sadiq (a.s) about the meaning of ‘infallibility in the realm of humans and he replied:
“The Infallible means he who by the will of God abstains absolutely from all that is forbidden. Indeed Allah the almighty said: Whoever holds firmly to Allah, then he is indeed guided to the Right Path.”.
Therefore ‘infallibility` in the realm of humankind means the infallible person enjoys a divine inspiration, by which he voluntarily yet absolutely protects (al-Isma) himself against any sin or error. Thus, practically, it is impossible for a Ma`soum to commit a sin or make a mistake.
A very liberal example for understanding the infallibility is when you adapt a good habit as your second nature. For instance, it`s become very natural for you to refrain from drinking human urine. It is your insight and certain knowledge about the harmfulness of the urine that not only protects you from drinking it, but even to hear about it sounds disgusting. Thus if you enjoy the insight into, for example, the harms of drinking alcohol, the same behaviour will be observed.
 Al-Mofradaat by Al-Raghib Al-Isfahani
 Beharul-Anwaar, vol. 25, p. 194 & Chapter 3 Ayah 101 of the holy Quran.
Sheikh Mansour Leghaei