Istishab (Arabic: الإستصحاب) is one of the most important and oft-used principles in Usul al-Fiqh. It is one of the four major Procedural Principles that are used by jurists in a case in which a person has "a previous certainty" and "a present doubt" about the same thing. It is often translated as "the presumption of continuity."
Istishab comes from the same morphological route as the word sahib, meaning companion. Istishab literally means "seeking companionship." Juristically, istishab applies in a situation where a person was in a state of certainty about a certain state of affairs, and then doubts whether or not that state of affairs still holds true. The principle dictates that a person assume that state of affairs still holds true. For example, if a person knew that a piece of clothing was clean, and then doubted whether or not it was still clean, they would assume that it is still clean. Because this principle can see application in any subject matter of Islamic law, it is studied as part of usul al-fiqh, rather than al-qawa'id al-fiqhiyyah.
Istishab is usually said to have four "pillars", which are the conditions required for it to go into effect:
1) Past certainty (اليقين بالحدوث): The person must have certainty about the past state of affairs. This does not necessarily mean that they must have actual, epistemological certainty. The past state of affairs must be confirmed either by direct certainty, or by some evidence that is assigned legal value in Islamic law. For example, if two just witnesses (whose testimony is accepted in Islamic law) bore witness that a piece of clothing was ritually clean, and then a third person doubts if the thing is still clean, they would still assume that it is clean
2) Doubt in continuity (الشك بالبقاء): The person must be in doubt about the continuity of the state of affairs. Istishab is an apparent ruling, meaning that it is a ruling that only operates when a person is in doubt. Following the previous example, if the person knew that their clothes were ritually clean and knows that they are no longer ritually clean, then there is no space for this principle to operate. It is also necessary that the time of doubt be connected to the time of certainty; if one had certainty their clothes were ritually pure, then knew they weren't, and then doubted if the clothes were still ritually impure, they could not use istishab of the original state of ritual purity.
3) Unity of the Proposition Concerning the Thing Known and the Thing Doubted (وحدة القضية المتيقنة و المشكوكة): What this means is that the state of affairs that was known before must be the same state of affairs that is doubted know. For example, a person knows that the clothes owned by Zayd are ritually pure, and he doubts if the clothes owned by 'Amr are ritually pure. There is no place for istishab here, since the thing that was known and the thing that is doubted are separate. The proposition must be the same, such as the proposition "Zayd's clothes are ritually pure." That proposition must be known to be true previously, and that same proposition has to be doubted now, for istishab to be applied and for Zayd's clothes to be still ruled ritually pure.
4) That the state of affairs assumed to be continued has a legal effect (كون الحالة السابقة في مرحلة البقاء ذال أثر مصجج للتعبد ببقائه). This means that istishab only applies in legal matters. If one knows, for example, that Zayd was wearing a green shirt today, and then one doubts if he is still wearing a green shirt, istishab would not apply, as there is no legal relevance to him wearing a green shirt. This is important, because most scholars of usul al-fiqh will argue that one cannot use istishab unless the state of affairs that one will assume is still continuing on the basis of istishab has direct legal value. For example: let us say that we have a large container of water in a house, large enough that it does not become impure by coming in contact with impurity (what is called kurr. The house is locked and nobody else can come in. We then start to doubt if some of the water has evaporated, and whether or not that water is still of such a quantity that it does not become ritually impure by coming in contact with impurities. We go into the house and we realize we don't know where the water is. We search and we eventually find a container of water.
Rationally, since we know that nobody else has come into the house and there's no possible way that somebody could have brought another container of water in, we would assume that this is the same water. By istishab, we would assume that it is still kurr, and so it would not become impure if it came in contact with something impure. However, the assumption that this is the same water is based on reason, and not based on any legal evidence. For us to do istishab of the previous water being kurr, we have to make this rational assumption. But because the thing that one is doing istishab with must have a direct legal effect, 'without recourse to a rational intermediary such as the one in the previous example. In other words, the legal effect of istishab has to be applied directly, without any kind of rational intermediary. When istishab is applied via some kind of rational or otherwise non-legal intermediary, it is known as al-asl al-muthbit (الأصل المثبت). The vast majority of Shi'ite jurists reject the efficacy of al-asl al-muthbit.
Scholars have differed about how to classify istishab. It has been mentioned that istishab is a hukm zahiri, and the hukm zahiri, which is a legal ruling that operates in a situation of doubt. It has two general types:
- An amarah (أمارة): An immarah is a hukm zahiri that operates on the basis of probability. The legal value of a reliable person's report is considered an immarah, since it operates under the assumption that reliable people normally give reliable reports.
- An asl (أصل): An asl is a hukm zahiri that is not based on probability. An example would be the principle that everything is ritually pure until proven otherwise. Obviously, most things are not always ritually pure; the probability has nothing to do with it, but rather the rule is made in order to give ease to people in their daily religious lives.
The question of where to categorize istishab has been a somewhat vexing one. It is argued by most that it is not an amarah, since the probability issue does not seem to play into it. If it is an asl, however, then it is a special one. For a normal asl delimits and defines the reality (واقع) of a given situation, but istishab is not considered to do this. It orders a person to act as if they have knowledge that the state of affairs still holds, without actually saying that the state of affairs 'does' still hold. An asl that defines the reality of a state of affairs is called a asl ihrazi (أصل إحرازي) or an asl muhriz (أصل محرز). Istishab is usually classified as an asl ghayr muhriz (أصل غير محرز) because of its intermediate state.
What this means in practice is that istishab takes precedence over an amarah, but does not take precedence over another asl. So, the principle of purity, which is an amarah, confirms that anything is ritually pure until proven otherwise. However, if a person knew from before that something was ritually impure, then istishab would apply. However, if it came in conflict with another asl, then that asl would take precedence. The reason for this is that an asl ihrazi defines the reality of a situation, and so negates the second pillar of istishab.
Types of Istishab
There are many other uses of istishab as well:
- Istishab al-Kulli (إستصحاب الكلي)
- Istishab as-Sababi wa al-Musabbabi (إستصحاب السببي و المسببي)
- Istishab al-Mawdu'at al-Murakkabah (إستصحاب الموضوعات المركبة)
- Istishab at-Ta'liqi (الإستصحاب التعليقي)
There are a number of legal evidences for istishab, and it is a principle that is used by all schools of law within Twelver Shi'ism.
The primary evidence is the hadith of Imam Jafar al-Sadiq, which is considered sahih and is narrated by Zurarah. He asked Imam Sadiq about whether or not a person who doubts if they have slept has to do wudhu again (since sleep invalidates wudhu):
لا حتى يستيقن انه قد نام حتى يجئ من ذلك امر بين والا فانه على يقين من وضوئه، ولا ينقض اليقين أبدا بالشك، ولكن ينقضه بقين آخر
No, nut until he is absolutely certain that he has slept. This is because he was certain that he was on wudhu, and certainty is never broken by doubt. Certainty is only broken by another certainty.
A secondary evidence is derived from sirat al-'uqalah, (the way of rational people). This principle is not based on a belief in the efficacy of natural reason in religious law. Rather, "the way of rational people" indicates something that rational people simply do. The fact that many people do something is not, in and of itself, a source of Islamic law. However, if it is known that rational people normally do a thing, and that this thing is a universal practice amongst all people, and there is nothing within the Qur'an or the statements of the Prophet or Imams prohibiting people from doing it, then this implies a tacit approval on the part of the lawgiver. It is said that rational people will normally assume that something is still the case until proven otherwise, and there is no prohibition on the part of the Lawgiver for this practice. Therefore, Islam gives its tacit consent to it, and it is something a person is allowed to act upon.