Question: If two men bring their case before one of the people of knowledge and appoint him to judge between them with the ruling of Allah regarding what lies between them of disagreement, but their problem is subtle and hidden, such that there is no specific ruling from Allah Most High and His Messenger (May Allah bless him and his family and grant them peace) regarding it, then what is the arbitrator to do?
Answer: The judge rules in such a matter according to what preponderates for him, if he is among the people of weighing [evidences]; and there is no way to attain more than presumption and preponderance in subsidiary ijtihād-based issues. The ruling of Allah Most High in practical subsidiary issues is divided into two types:
1. A type in which the ruling is known with certainty.
2. A type in which the ruling of Allah is thought by presumption.
And in all of them it is said that it is the ruling of Allah.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2