What is privacy? A good working definition is that privacy is the right to control who knows certain aspects about you, your communications, and your activities. In other words, you voluntarily choose who can know which things about you. People may ask you for your telephone number: your auto mechanic, a shop clerk, your tax authority, a new business contact, or a new friend. In each case, you consider why the person wants the number and then decide whether to give it out. But the key point is that you decide. So privacy is something over which you can have considerable influence.
Privacy is the right to control who knows certain things about you.
You do not have complete control, however. Once you give your number to a person or a system, your control is diminished because it depends in part on what the person or system does with that information. In giving out your number, you are transferring or ceding authority and control to someone or something else. You may say “don’t give my number to anyone else,” “use discretion,” or “I am sensitive about my privacy,” but you do not control the other person or system. You have to trust the person or system to comply with your wishes, whether you state those wishes explicitly or not. This problem is similar to the propagation problem of computer security: Anyone who has access to an object can copy, transfer, or propagate that object or its content to others without restriction. And even if you specify that the object should be deleted or destroyed after a certain period of time, you have no way to verify that the system or person really does destroy the content.