Machine summary:
"Reason, Faith & Authority: A Shi‘ite Perspective Mohammad Ali Shomali From a Shi‘a point of view, there are four sources on which any investigation about Islam has to be based: the Glorious Qur’an, the Sunnah (including sayings, actions and tacit approval of the Prophet), reason, and consensus.
The Shi‘a believe that by the exercise of reason every person can come to understand that God exists, that He has sent certain people as His messengers, and that Resurrection will take place.
Of course, each may imply the other secondarily and in practice a faithful person is the one who both believes that God exists and trusts Him. However, the question is which one primarily constitutes the faith Reflecting on relevant verses of the Glorious Qur’an, normally the assumption among philosophers of religion is that in Islam faith has a propositional nature.
Among those who believe in basic religious truths and commit themselves to those truths, some people may be prepared to commit and submit themselves just in declaration of faith, some may be prepared to fully or partly practice their faith and some may be prepared to submit their entire reality including acts, heart and mind to God. Faith may also have degrees according to different degrees of the required knowledge or different degrees of the consequent acts.
57) 12 Despite some differences in positions of the Shi‘a and the Mu‘tazilites, they are both called "ahl al-‘adl" (the people of justice), because they both believe in independent moral values and in the existence of rational criteria for judging what is good and what is bad, and because a proper defence of the principle of divine justice depends on the belief in independent and rational good and evil."