Q. Last week the office ran out of coffee. One coworker has a private jar, but he was nowhere to be found. Can I borrow a spoonful?
A. This is a common issue, and many people see nothing wrong with "borrowing" from a friend in this way. In order to get to the bottom of this issue, we have to start from the proper foundations. The Talmud tells us a basic principle of ownership: "Borrowing without knowledge [of the owner] is considered stealing." (1)
The basis for this pronouncement is clear. Whether something is considered stealing or borrowing doesn't depend on the opinion of the taker; he has in any case no rights in the object. It can only depend on the opinion of the owner. Since the owner doesn't know of the "borrowing", the borrower's subjective intention to return the object has no power to change the legal status of the taking.
However, there can be cases where it is so clear that the person wouldn't mind that it can just be assumed. One example is a mitzvah object. In the case where borrowing would help a person do a mitzvah (religious commandment), and where the object is not harmed at all, our sages say, "A person agrees to have his property used for a mitzvah". (2) However, even this principle does not apply if there is a reasonable chance that damage will come to the object, for example borrowing a book which may become torn. (2)
Another example is an object whose whole purpose is to serve strangers. If you invite someone into your house, the guest generally doesn't need to ask permission to sit on the couch – that's what it's there for. (3)
At the same time, the rabbis of the Talmud were well aware of the danger of rationalization. It is just too easy to convince yourself that the owner surely doesn't mind. This tendency can be an expression of an exaggerated sense of entitlement which expresses itself in even worse ways. Consider the following story form the Talmud:
A silver cup was stolen from a boarder of [the sage] Mar Zutra Chasida. He saw a certain student who washed his hands and then dried them on his fellow's cloak. He exclaimed, This must be the person, see how he has no regard for his fellow's property! He took him aside and he confessed.(4)