his own action, while he cannot have that regarding the action of someone else. Thus, they differ regarding the oath, just as testimony differs; for it is given with certainty in matters where certainty is possible among contracts, and based on conjecture in matters where certainty is not possible among properties and lineages, and based on the negation of knowledge in matters where encompassing it by way of negation is impossible, such as testifying that a person has no heirs other than so-and-so. The hadith of al-Qasim ibn 'Abd al-Rahman is understood as referring to an oath regarding the negation of another person's action. Once this is established, he swears regarding what is against him with certainty, whether it is a negation or an affirmation. As for what pertains to the action of another, if it is an affirmation, [such as if he claims that he acknowledged or sold, and establishes a witness for that, then he swears along with his witness upon certainty and decisiveness. And if it is upon the negation of knowledge] (8), such as if a debt, usurpation, crime, or betrayal (9) is claimed against him, then he swears upon the negation of knowledge, and nothing else. If he swears regarding it upon certainty, it suffices him, and the intended meaning in it is knowledge, as in the case of a witness when he testifies to the number of heirs and says: He has no heirs other than them. This is accepted, and the intended meaning in it is his knowledge. If it is claimed against him that his slave committed a crime or incurred a debt, and he denies it, his oath is upon the negation of knowledge; because it is an oath upon the negation of another's action, so it resembles the heir's oath upon the negation of the deceased's action.

Section: Ibn Abi Musa said: There is a difference of opinion attributed to Ahmad regarding someone who sold an item, then the buyer discovered a defect in it, and the seller denied it: is the oath based upon certainty or upon his knowledge? There are two narrations. If the buyer's slave ran away, and he claimed against the seller that he had run away while with him, and the seller denied it, is he obligated to swear that he never ran away, or upon the negation of his knowledge? There are two narrations, unless it was his child, [then he is obligated to swear] (10) that he never ran away. The reasoning for the oath being upon (11) his knowledge is that it is upon the negation of another's action, so it resembles the case where it is claimed against him that his slave committed a crime. The reasoning for the other [view] is that when he (12) claims against him that he sold it to him while defective, he is entitled to return it to him, so the oath is incumbent upon him with certainty, just as if it were an affirmation.

Section: Whoever has an oath directed against him in which he is truthful, or it is directed in his favor, it is permissible for him to swear,