In December 1890 two Boston lawyers, Samuel Warren and Louis Brandeis, published an article in the Harvard Law Review called "The Right to Privacy." What alarmed them was new technology. Cheap portable cameras had just reached the market, and newspapers had learned that gossip about the private lives of prominent families sold copies. They wrote that "instantaneous photographs and newspaper enterprise have invaded the sacred precincts of private and domestic life." Their answer was to argue that the common law already protected something they called, borrowing a phrase from the judge Thomas Cooley, the right to be let alone. More than a century later the cameras are in every pocket and the gossip sheets have become data companies, but their question is still the right one: what may others know about us, and who decides?

Three ways to define privacy

There is no single agreed definition, and it helps to hold three ideas side by side. The first is seclusion: privacy as a space where others cannot intrude, like a closed door or a sealed letter. This is the idea Warren and Brandeis started from.

The second idea treats privacy as control over information about yourself. In his 1967 book Privacy and Freedom, the political scientist Alan Westin described privacy as a person's claim to decide when, how and how far facts about them are passed on to others. On this view, privacy is not secrecy. You may happily tell your friends about your illness and still be wronged if your employer finds out.

The third idea, developed by the philosopher Helen Nissenbaum, is called contextual integrity. Every flow of information happens in a context with its own norms. Telling your doctor about your symptoms fits the norms of medicine; the same facts flowing to an advertiser break them. A privacy violation, on this view, is information moving in a way that breaks the norms of the context where it was shared. This explains something the other views struggle with: why it can feel wrong when a company collects facts that are technically "public," like where you walk each day.

Why it matters

Privacy protects room to think and change. People try out ideas, beliefs and identities before they are ready to defend them in public. A teenager questioning a religion, an adult considering a divorce, a worker thinking about forming a union: each needs a space where half-formed thoughts are not yet on the record.

Privacy also makes relationships possible. We share different things with a spouse, a doctor, a coworker and a stranger, and the fact that we can choose what to share is part of what makes closeness meaningful. The philosopher Charles Fried argued that love, friendship and trust all depend on this ability to give some people what we withhold from others.

Finally, privacy protects democracy. The secret ballot exists so that voters cannot be bribed or punished for their choices. In NAACP v. Alabama (1958), the US Supreme Court blocked the state of Alabama from forcing the civil rights group to hand over its membership lists, recognizing that members exposed to their neighbors and employers could face firing or violence. Freedom to associate, the Court saw, can depend on privacy.

The "nothing to hide" argument

A common reply to privacy worries is that if you have done nothing wrong, you have nothing to fear. At its strongest the argument says that honest people lose very little when they are observed, while wrongdoers lose the cover they depend on. That is a real point, and in some settings, such as audits of public spending, more transparency clearly serves the public.

The legal scholar Daniel Solove has offered the best known reply. The argument, he says, assumes privacy is only about hiding bad deeds. Many privacy harms have nothing to do with guilt. One is aggregation: combining many harmless facts into a profile that reveals far more than any single fact. Another is exclusion: not knowing what is held about you, or being unable to correct a mistake. A third is the imbalance of power when one side sees everything and the other sees nothing. Rules also change: something legal and harmless today may be treated differently by a future government.

It is fair to add a criticism from the other side. Privacy can also shield wrongdoing. For generations, the idea that the home was a private sphere helped hide domestic violence, and feminist writers rightly pointed out that "family matters" were often a cover for abuse. Corporations and officials also invoke privacy to avoid scrutiny. The serious question is never "privacy or no privacy" but which flows of information, to whom, under what rules.

Worked example: a mood tracking app

Priya has been feeling low and tells her doctor, who prescribes medication and sends the prescription to a pharmacy. Every step fits the norms of medical care. Priya also downloads a free app to track her mood each day. Buried in its terms is permission to share "insights" with partners, and an advertiser learns that this user is likely depressed.

Priya has nothing to hide in the criminal sense, yet the harm is real. Seclusion is not the issue: she chose to type her moods. Control is: she did not meaningfully decide who would receive them. Contextual integrity gives the sharpest diagnosis, because facts shared in a health context have flowed into a marketing context. Aggregation makes it worse: combine her mood logs with her phone's visits to a clinic and her late night purchases, and a stranger knows more about her than her family does. Seeing the problem clearly is the first step toward asking what rules should stop it.