I have heard Ijma & Qias are applicable if there is nothing specific in The Quran or Hadith.
Is that true??
So my question is what are Ijma and qiyaas, and when are they applicable?
Ijma is an Arabic word referring to consensus of the Muslim community.
The hadith "My ummah will never agree upon an error" is usually cited for the validity of ijma. There are a number of different opinions on who is part of the ijma, with most views split into two following two possibilities:
In addition, schools of thought differ on whether this consensus is to be that of the first generation of Muslims only, that of the first three generations of Muslims (i.e. the Salaf), the consensus of the jurists and scholars of the Muslim world (i.e. scholarly consensus) or the consensus of all the Muslim world, scholars and laymen alike.
It would be applicable when there is a problem that needs discussion, but a ruling is not directly present in either the Qur'an or sunnah; the Qur'an and sunnah are both still used to deduce the final ruling.
Qiyas is used to analyze and form a ruling, Ijma' and Qiyas are connected.
See also the relevant wikipedia page here: Ijma
Ijmaa and Qiyaas are two technical terms from usool ul-fiqh, or the science of "how do we derive fiqh?" In short, scholars use proofs in the following order (which denotes their strength over lower-level proofs):
After that, each madhab differs over what types of proof dominate over what types of proof.
For example, in the issue of "is photography allowed in Islam," there's obviously no ayah of Qur'an or hadith that says "cameras/photography blah blah blah." Ditto for ijmaa. Scholars then use qiyaas to address it; some say "snapping a photograph is like drawing or making a statue," and others say "no, it's digital so it's not the same at all."
Ijma means consensus. Normally it is regarded as "consensus among the all muslims". But some other opinion confined it to "consensus among all ulema". And some other say it is only "consensus among the companions of our prophet". Ijma is itself a source and need not to be derived from Quran and sunnah.
In the contrary, Qiyas or analogy is dependent on Quran and sunnah. When a new situation arises which do not have clear guideline in the Quran and sunnah, qiyas is used to give the ruling by analyzing similar rulings in Quran and sunnah. So, this one is derived. And there may be different qiyas from different scholars or ulema for the same situation.
These terms deal with how laws are formed in Islam.
Ijma means consensus of the community. Initially whatever the whole Muslim community decided as a group (after the death of Prophet Muhammad(pbuh)) becomes part of law. Today, in practice, however this is now consensus of the scholars of the community (since the Muslim community is way too big to arrive at a consensus). For example, there is an ijma that consuming intoxicating drugs is haram. The theological basis for this method of arriving at laws is based on, among other things, on the hadith "My community will never agree upon an error."
Qiyas is a method of deductive analogies with known laws which is often used to form an ijma on some issue not mentioned in the Quran or the hadith. For example, alcohol is haram on the basis of Quran. Now, analogously intoxicating drugs have the same effect and should be haram too. They were not known to 7th C Arabs and thats why they were not directly prohibited. Hence by the method of analogies or Qiyas, it may be concluded that intoxicating drugs are haram too. When all scholars agreed that this analogy is valid then an ijma was achieved and this became part of Islamic law.