About the soul

By the sun and its morning brightness! By the moon when it follows the sun! By the day when it illuminates (everything)! By the night when it enshrouds the day! By the heaven and He who built it! By the earth and He who extended it! And by the soul and He who perfected it! Then He inspired it to understand what is good and what is evil. Prosperous is he who purifies it, and failed is he who seduces it. (Qur'an 91:1-10)

After swearing by the most majestic signs of His creation, Allah says that the pure human soul has the ability to understand what is right and what is wrong provided it is purified and uncorrupted. This verse makes it clear that the human soul, just like his body, is capable of becoming spiritually impure and unclean. Imam 'Ali (as) has said, `The human soul is a precious jewel; whoever protects its, enhances its (effectiveness), and whoever degrades its, decreases its (effectiveness)."

al-Amudi, Ghuraru 'l-hikam, p. 226

The impurities that can corrupt a human soul are collectively known as "sins". Accumulation of sins can indeed render the human soul ineffective and, in Qur'anic expression, `seize the heart'. Allah says, "Whatever (sins) they have committed has seized their heart." (83:14) By committing sins, not only is the soul of a Muslim seized but he also spiritually turns away from Allah. Sins create a distance between God and man.

Can a person rescue his soul from the seizing of the sins? Can a sinner spiritually get closer to God? Yes, indeed, a sinful person can spiritually return to Allah. Returning to Allah means repenting and asking forgive­ness for your sins. Imam Muhammad al-Baqir (as) has explained this phenomenon as follows: "Each believer has a bright soul. When he commits a sin, a dark dot appears on his bright soul. If he repents, the dark dot will disappear. But if he persists in his sins, the darkness will increase until it covers the entire soul-then the person would never return towards goodness."

al-Majlisi, Biharu 'l-Anwar, vol. 73, p. 361

Taken from http://www.al-islam.org/ritualandspiritual/ 

A very good read about ritual and spiritual impurity.