Imam Ali (peace be upon him), in his argument against an unbeliever who asked him about the ambiguous verses of the Quran, said: "...[Imam Ali (peace be upon him)] answered him: Allah has appointed worthy ones for knowledge and, through this saying of His, has made obedience to them obligatory upon the servants: 'Obey Allah, and obey the Messenger of Allah and the ulul-amr (the successors of the Prophet)!' (al-Nisa/59), and through this saying of His: 'Whereas if they had referred it to the Messenger and to the leaders who have sufficient power of discernment, those among them who draw conclusions would have known it.' (al-Nisa/83), and through this saying of His: 'Refrain from disobeying Allah's command, and be with the truthful!' (al-Tawbah/119), and through this saying of His: وَ ما یَعْلَمُ تَأْوِیلَهُ إِلَّا الله وَ الرَّاسِخُونَ فِی الْعِلْمِ [And none knows its interpretation except Allah and those firmly rooted in knowledge], and through this saying of His: 'And enter the houses through their doors.' (al-Baqarah/189) — and these houses are the houses of knowledge which the prophets (peace be upon them) placed as a trust, and its doors are their successors. So every act of the acts of good that is carried out by other than the successors, and is not performed in accordance with their covenants, their limits, their laws, their ways and the marks of their religion, is rejected and unacceptable, and its people — even if the description of faith encompasses them — are in the station of disbelief. Then Allah divided His speech into three parts: one part He made such that both the learned and the ignorant know it; and a part which none knows except one whose mind is clear and pure, whose perception and understanding are precise, and whose power of discernment is sound — and these are among those whose hearts Allah has opened to Islam; and a part which none knows except Allah, His angels and those firmly rooted in knowledge. And Allah assigned a part of the knowledge of the Book exclusively to this group so that the people of falsehood, who have laid hold of the inheritance of the Messenger of Allah (peace be upon him and his family) — which is the knowledge of Allah's Book — cannot claim what Allah has not appointed for them, and so that they would be compelled to follow the one who is the guardian of their affair; yet they, out of arrogance, did not obey him."