Every specific Islamic rule originates from a law maker. Here, we investigate who can say, in Islam, that something is the law.
Allah (سبحانه وتعالى), Lord of the Worlds, is the law maker. None can dispute His laws. What’s the proof? In Surah Yusuf Allah (سبحانه وتعالى) says:
Subhanallah, the basis of Western law is that intelligence is the most trustworty, and the source of all law. But is that true in Islam? Can Intelligence be the law maker?
Surah Nisaa reveals the answer:
Translation: Apostles who gave good news as well as warning, that mankind, after (the coming) of the apostles, should have no plea against Allah: For Allah is Exalted in Power, Wise. [Surah Nisaa, verse 165]
Intelligence is used in making laws, but it is not the law maker. Check the ayah above: if people never get the message, they say “nobody told me”; Allah (سبحانه وتعالى) does not ask, “Why didnt you figure it out yourself?” Because that method will never lead to the true Law. (Just look at Western socities, and where their laws, influenced as they are from Islam, are now.)
What’s more, ‘Umar ibn al-Khattab (رضي الله عنه), who was the most intelligent among the companions, only about four times reached a conclusion that matched a command of Allah.
This confuses a lot of Muslims. Why do we accept law from the Prophet? Well, because of what Surah Al-Ghashiyah:
لَّسْتَ عَلَيْهِم بِمُصَيْطِرٍ
Translation: Therefore do remind, for you are only a reminder. You are not a watcher over them. [Surah Gashiya, verse 21-22]
If the Prophet (صلي الله عليه وسلم) is not a law maker, what does that make a mujtahid or an imam?
They are not law-makers. They simply research (inshaAllah) and tell you what they think Allah told us to do. Based on their research and tools (such as codes of Usool-ul-Fiqh).
It’s sad that, in our community, people say things like “the shaykh said mortgages are ok” and when you show them ayat probiting interest from the words of Allah, they turn away. Or worse, they argue (one of the characteristics of munfaiqin). What are they really saying? “Well, my Mufti says so, who cares about the Qur’an?” May Allah protect us from being among such people, ameen.
If you know such people, remind them (gently) of the hadith of the Prophet (صلي الله عليه وسلم): “There is no obedience to the creation when it entails disobedience to the Creator.” [Musnad of Ahmad] 
There is also this hadith of the Prophet:
The Prophet sent a Sariya under the command of a man from the Ansar and ordered the soldiers to obey him. He (i.e. the commander) became angry and said “Didn’t the Prophet order you to obey me!” They replied, “Yes.” He said, “Collect fire-wood for me.” So they collected it. He said, “Make a fire.” When they made it, he said, “Enter it (i.e. the fire).” So they intended to do that and started holding each other and saying, “We run towards (i.e. take refuge with) the Prophet from the fire.” They kept on saying that till the fire was extinguished and the anger of the commander abated. When that news reached the Prophet he said, “If they had entered it (i.e. the fire), they would not have come out of it till the Day of Resurrection. Obedience (to somebody) is required when he enjoins what is good.” [Bukhari 5/59/629]
Subhanallah. May Allah protect us from misguidance and from the Fire, ameen!
 Abdul Hasib. “A Trip to Your Neighbour.” There is no obedience to the creation when it entails disobedience to the Creator. 6 Aug. 2006. 13 Aug. 2006 <http://forums.almaghrib.org/showthread.php?t=9343&page=15&pp=10>.