This is an important question. Adherence to every halachic nuance is as important in day-to-day monetary and personal relations as it is in the most famous ritual laws.
The matter starts with the sugya[1] of yei’ush shelo mida’at (loss of hope without knowledge) in Bava Metzia.[2] It concerns one who loses an object under circumstances where, upon finding out that it is lost, will despair of recovering it. Can someone who found the object before the owner became aware of the loss keep it? The gemara tries to derive an answer from the following story: Three rabbis visited Mari bar Isak’s orchard, and his sharecropper brought them fruit from the orchard. Mar Zutra refused to eat because Mari was not there to give permission, although presumably he would have wanted the honored guests to have the fruit.[3] Thus, it is evident that the critical factor is the owner’s conscious thoughts, not his presumed intentions were he to find out. Tosafot[4] say that the other rabbis actually agreed with Mar Zutra in principle, but assumed that the sharecropper had given them fruits from his own portion. According to Tosafot’s approach, indeed, one may not depend on the permission the owner would have given had he known the situation.
Others disagree with Tosafot. The Ran[5] cites the Rashba, who says that when one can assume that the owner would be happy to share his food with certain guests, they may eat without his knowledge. He cites talmudic precedents where we assume that a homeowner has in mind to authorize others to give on his behalf. The Shach[6] distinguishes between this case and yei’ush on a lost object. A person would prefer not to have yei’ush. Therefore, only if he actually loses hope does he lose the object. In contrast, in the event that the owner would want to share with his unknown guest, the latter may partake without the owner’s knowledge.
There is not a consensus among today’s poskim on which position to accept.[7] Therefore, it is preferable not to take a friend’s object without permission. This certainly applies to food, which is consumed and lost to its owner. Although reimbursement mitigates the situation, it does not erase the fact that, according to Tosafot, his action was theft (excuse the harsh term). Even in regard to objects that can be borrowed and returned, one who borrows without permission is a thief.[8] However, we cannot condemn one who relies on the Shach’s leniency.
In certain cases, the owner’s explicit permission is not required. One can give blanket permission for those close to him to take things without permission, which may even be assumed regarding some neighbors and guests during their stay. It is permissible to use an object of so little value that owners generally do not care if anyone uses it.[9] If, under normal circumstances, someone receives an object from a responsible member of a household, he can assume that the member has explicit or tacit authority to take such action on behalf of the heads of the household. For that reason, tzedaka collectors may accept modest donations from older children at the door without knowing the family situation.[10] In a setting in which it is clearly accepted for people to borrow certain things without permission, one can assume that it applies to any given person. For example, in many yeshivot there is a policy of borrowing another’s sefarim for short periods of time. If a talmid does not want people to borrow his sefarim, he should put such instructions in writing on or in the sefarim to save others from doing wrong unknowingly. Regarding articles to be used for mitzvot, permissibility of borrowing depends on the article in question and other factors, which likely change depending on time and place. Details are beyond our present scope.
Let us caution the potential borrower: It is wrong to assume, “I would give him, so he would give me.” People have different natures and upbringings. Aside from halachic concerns, many relationships between siblings and friends have been strained by incorrect assumptions of this sort. As it says in Pirkei Avot, “One who says… ‘Mine is yours, yours is yours’ – is a righteous person.” [11]