Jurists and scholars of legal theory divide uncertainty into two: 1) those that concern a law (al-shubha al-ḥukmīyyah), and 2) those that concern the subject or an instance of a law (al-shubha al-mawḍū’īyyah). Perhaps the simplest way these two uncertainties can be explained is through the following three perspectives:
1. The Root of Uncertainty
The root cause of al-shubha al-ḥukmīyyah is one’s ignorance with respect to the legislation of a law. In other words, one does not know what law the Legislator has ordained for him or her. As far as one’s access to the legislated laws is only possible through the Legislator’s own words, one can say that the origin of al-shubha al-ḥukmīyyah is rooted in the evidence that we have in our hands. Some of this evidence could be lost, some of it could be ambiguous, or some of them are contradictory.
Al-Shubha al-Ḥukmīyyah itself can be further divided into three:
a) Ignorance of the law itself: An individual does not know what type of law God has legislated; for example, one does not know whether burying a deceased hypocrite has a ruling of obligation (wujūb) or prohibition (ḥurmah).
b) Ignorance of what the law is connected to (muta’allaq): An individual does not know to what action a law is connected to; for example, one does not know if wujūb has been legislated for the Friday prayers or for Ẓuhr. In this scenario, what the law (i.e. obligation) is connected to is unknown. Amongst contemporary scholars of legal theory, muta’allaq of a ḥukm is that action to which a law is connected to.
c) Ignorance of the subject-matter of a law: An individual does not know under what conditions the Legislator decided for there to have been a law; for example, one does not know whether permissibility of lying in an emergency is conditioned to incapability or only if it is a black lie (tawrīyah).
It is clear that in all these cases, the uncertainty of an individual is related to the realm of legislation. They either do not know what law the Legislator has legislated, or for what action it has been legislated upon, or under what conditions a law actually exists. However, the cause of al-shubha al-mawḍū’īyyah is an individual’s ignorance to the instance of a legislated law. This itself is divided into two:
a) Ignorance of whether the subject-matter has been realized: An individual does not know whether the subject-matter of a law (i.e. conditions that a law’s application is based on) has been realized in external reality or not; for example, one does not know whether they have become capable (mustaṭī’) or not.
b) Ignorance of whether the muta’allaq of a subject-matter has been realized: An individual does not know whether the muta’allaq of a law has come into existence in external reality or not. For example, they may not know whether they performed a pilgrimage during their youth or not, or whether they performed it correctly or incorrectly.
In other words, in al-shubha al-mawḍū’īyyah an individual does not know whether a subject-matter of a law, or what it is connected to, has an instance in external reality or not. This is why sometimes al-shubha al-mawḍū’īyyah is also called uncertainty concerning an instance (al-shubha al-miṣdāqīyyah). This term is in contrast to uncertainty concerning a concept (al-shubha al-mafhūmīyyah), which is a type of al-shubha al-ḥukmīyyah, since it refers to a case where an individual does not know what the Legislator meant by a specific concept that has been taken into consideration during legislation. For example, an individual may not know whether the term ghinā in the evidence al-ghinā ḥarāmun is a concept that refers to a vibrant sound, or a melodious sound, or both?
2. Universality and Particularity
The author has mentioned numerous times that al-shubha al-ḥukmīyyah is universal, and al-shubha al-mawḍū’īyyah is particular. For example, uncertainty regarding the impurity of alcohol is al-shubha al-ḥukmīyyah, and uncertainty regarding the impurity of a specific cloth is al-shubha al-mawḍū’īyyah. Even though this criterion is true for the most part, it is not a universal rule. This is because sometimes al-shubha al-mawḍū’īyyah can also be universal. As an example, one may doubt whether sharks have scales, or doubt what responsibility a hermaphrodite has.
Likewise, it is rationally possible to imagine al-shubha al-ḥukmīyyah which is particular. For example, it is possible for God to legislate a specific law for Zayd with respects to one of His (swt) Prophets, and Zayd is uncertain as to what this law is.
3. Acting upon Indications (Amārāt) and Procedural Principles (al-Uṣūl al-‘Amalīyyah)
In both cases of uncertainty, an individual is first responsible for referring to all non-certain amārāt, and if relevant ones are absent, they are to refer to al-ūṣūl al-‘amalīyyah. However, the difference is that in al-shubha al-ḥukmīyyah the jurists have a consensus that before acting on an amārah it is obligatory to investigate whether a contradicting amārah exists or not, and before applying a procedural principle they consider it obligatory to exhaustively investigate whether an amārah exists or not. This is to ensure that there is no evidence that might exist which contradicts with an amārah or prevents us from applying a procedural principle.
Given this, before acting on a report of a trustworthy narrator, one must investigate all reliable reports and only after attaining despair with respects to finding contradictory evidence, one can give an edict based off of the report. Since such exhaustive investigation into contradictions and as well as looking into detailed evidence can only be carried out by a jurist, applying amārāt and procedural principles in al-shubha al-ḥukmīyyah is only permissible for a jurist.
As for al-shubahāt al-mawḍū’īyyah, jurists do not consider it obligatory to look for contradictory reports in order to act on an amārāh, and neither do they consider looking for an amārāh obligatory in order to apply a procedural principle. Therefore, if two testimonies or a procedural principle is established with respects to the purity of a cloth for an individual, they can act in accordance to that without any delay. They do not need to further research or look into whether there exists another amārah which might suggest the cloth is impure.
Thus, since no specific investigation with respects to amārāt and procedural principles is required for al-shubha al-mawḍū’īyyah, referring to them is not restricted to the jurists, but rather even their followers can refer to them on their own.
This article is a summary of lessons given by Sayyid Muḥammad Ṣādiq ‘Alam al-Huda. The original post in Persian can be found here: https://t.me/Alamolhoda_org/209
 Farā’id al-Ūṣūl, vol. 2, pg. 18-19