A philosophical perspective:

But while Descartes’s overall view has been rightly rejected, there is something profoundly right about the connection between privacy and the self, something that recent events should cause us to appreciate. What is right about it, in my view, is that to be an autonomous person is to be capable of having privileged access (in the two senses defined above) to information about your psychological profile ­ your hopes, dreams, beliefs and fears. A capacity for privacy is a necessary condition of autonomous personhood.

To get a sense of what I mean, imagine that I could telepathically read all your conscious and unconscious thoughts and feelings — I could know about them in as much detail as you know about them yourself — and further, that you could not, in any way, control my access. You don’t, in other words, share your thoughts with me; I take them. The power I would have over you would of course be immense. Not only could you not hide from me, I would know instantly a great amount about how the outside world affects you, what scares you, what makes you act in the ways you do. And that means I could not only know what you think, I could to a large extent control what you do.

That is the political worry about the loss of privacy: it threatens a loss of freedom. And the worry, of course, is not merely theoretical. Targeted ad programs, like Google’s, which track your Internet searches for the purpose of sending you ads that reflect your interests can create deeply complex psychological profiles — especially when one conducts searches for emotional or personal advice information: Am I gay? What is terrorism? What is atheism? If the government or some entity should request the identity of the person making these searches for national security purposes, we’d be on the way to having a real-world version of our thought experiment.

But the loss of privacy doesn’t just threaten political freedom. Return for a moment to our thought experiment where I telepathically know all your thoughts whether you like it or not From my perspective, the perspective of the knower — your existence as a distinct person would begin to shrink. Our relationship would be so lopsided that there might cease to be, at least to me, anything subjective about you. As I learn what reactions you will have to stimuli, why you do what you do, you will become like any other object to be manipulated. You would be, as we say, dehumanized.