This is one of the fundamental principles of legal deduction, even if it does not have a wide latitude like other principles. In general, it is a negative, not a positive principle, i.e. certain judgements arise from it, not by legal affirmation with confirmed evidence in which judgements are established by the lack of the existence of contrary established evidence which is different from the established state before.
Ibn al-Qayyim defined it as being the continuation of what is established or the negation of what does not exist, i.e. it is the judgement, negative or positive, continues until there is evidence of a change of state. This continuance is not proved by positive evidence, but by the absence of the existence of new evidence. Al-Qarafi defined it: "Istishab means the belief that the past or present matter must be assumed to remain as it is in the present or future."
This means that the past judgement and the knowledge of it makes one assume that it will continue in the future, like the one for whom ownership is affirmed by something like purchase or inheritance. So ownership continues until there exists something to negate it. It is also like someone who is known to be alive at a specific time. It remains probable that he is still alive until evidence is established to the contrary and something establishes his death. So an absent person is judged to be alive until there is something to indicate he has died and then the qadi judges him to be dead.
Al-Qarafi said: "Istishab was considered a proof by Malik as well as the Shafi'i, al-Muzani." He mentioned that he differed from the Hanafis in that. Then he mentioned that the evidence that it is a proof is that it it probable that an existing state will continue to exists until there is something to negate it. Such probability is evidence in action: like testimony. It is a binding proof for all. If it were ignored or not acted upon, rights would be lost since there would be no means to establish them.
According to this, istishab was considered proof by Malik as long as there was no evidence to contradict it. When a person is absent and it is not known whether he is alive or dead, he is considered to be alive until the Qadi judges that he is dead and he is deemed to be alive in the period between the absence and being judged to be dead.
Al-Qarafi mentioned that the Hanafis differ from the Malikis in that and some of them do not consider istishab to be a proof in its own right. However presumption of innocence is a firm principle which is relied on. It is like that when ownership is affirmed. It only ceases by a eliminating cause. All of this is involves the presumption of the continuation of the state. So most Hanafis who disagree with them say that continuation of the state is a defensive proof and not evidence of affirmation. That is why they permit a settlement after denial even though the claimant takes a reimbursement when the right has not been established. If istishab had been a proof which obliged rejection and affirmation, that settlement would not have been permitted as long as there was no evidence. So the evidence of the ownership of one against whom the claim is made would be affirmed by the principle of the continuation of the state, but the Hanafis, who permit the settlement, said that denial interferes with the principle of innocence. As they both have a right, each of them makes a settlement for a right permissible in respect of him.
They explain it as meaning a negative rather than affirmative presumption. It denies the entitlement of something against him.
Some scholars divided istishab into two categories:
1. Presumption of innocence. It is the continuance of inviolability until there is evidence which establishes a right, like the state of the one who denies a claim. His state is that of presumption of innocence. Ibn al-Qayyim mentions the dispute of the fuqaha' in it, saying that the Hanafis apply it to denial rather than affirmation. Malik, ash-Shafi'i and Ibn Hanbal accept it as absolute proof.
2. The continuity of the attribute. A judgement continues until its opposite is affirmed. Ibn al-Qayyim said that it is a proof about which the fuqaha' do not argue, but we disagree with Ibn al-Qayyim. The Hanafis said that the continuity of the attribute is a negative rather than affirmative proof of denial, i.e. that the attribute affirms the continuity of the condition, but it does not affirm a new right by it.
The summary of the position is that Malik, may Allah be pleased with him, used istishab as a proof and al-Qarafi, Ibn al-Qayyim, and others postulated a difference between him and the Hanafis, but the one who studies the secondary rulings of the two schools will find that both of them do not differ much from one other in the nature of the proof of istishab and the amount in which it is used as evidence. You will see that they are unified in the principle of istishab regarding the life of someone who is missing and make it affirm what was affirmed first but it does not establish a new right. They differ from ash-Shafi‘i in that.
The summary of the position is that Malik, may Allah be pleased with him, accepted istishab as a proof and although al-Qarafi, Ibn al-Qayyim, and others postulated a difference between him and the Hanafis, a study of the secondary rulings of the two schools shows that both of them do not differ much from one other in the nature of the proof of istishab and the amount in which it is used as evidence. You will see that they are unified in the principle of istishab regarding the life of someone who is missing and make it affirm what was affirmed first but it does not establish a new right. They differ from ash-Shafi'i in that.