In my book Data and Goliath, I write about the value of privacy. I talk about how it is essential for political liberty and justice, and for commercial fairness and equality. I talk about how it increases personal freedom and individual autonomy, and how the lack of it makes us all less secure. But this is probably the most important argument as to why society as a whole must protect privacy: it allows society to progress.

We know that surveillance has a chilling effect on freedom. People change their behavior when they live their lives under surveillance. They are less likely to speak freely and act individually. They self-censor. They become conformist. This is obviously true for government surveillance, but is true for corporate surveillance as well. We simply aren’t as willing to be our individual selves when others are watching.

Bruce Schneier is an internationally renowned security technologist. He teaches at the Harvard Kennedy School, and serves as special advisor to IBM Security. His new book is called Click Here to Kill Everybody: Security and Survival in a Hyper-Connected World.

Let’s take an example: hearing that parents and children are being separated as they cross the U.S. border, you want to learn more. You visit the website of an international immigrants’ rights group, a fact that is available to the government through mass internet surveillance. You sign up for the group’s mailing list, another fact that is potentially available to the government. The group then calls or emails to invite you to a local meeting. Same. Your license plates can be collected as you drive to the meeting; your face can be scanned and identified as you walk into and out of the meeting. If instead of visiting the website you visit the group’s Facebook page, Facebook knows that you did and that feeds into its profile of you, available to advertisers and political activists alike. Ditto if you like their page, share a link with your friends, or just post about the issue.

Maybe you are an immigrant yourself, documented or not. Or maybe some of your family is. Or maybe you have friends or coworkers who are. How likely are you to get involved if you know that your interest and concern can be gathered and used by government and corporate actors? What if the issue you are interested in is pro- or anti-gun control, anti-police violence or in support of the police? Does that make a difference?

Maybe the issue doesn’t matter, and you would never be afraid to be identified and tracked based on your political or social interests. But even if you are so fearless, you probably know someone who has more to lose, and thus more to fear, from their personal, sexual, or political beliefs being exposed.

This isn’t just hypothetical. In the months and years after the 9/11 terrorist attacks, many of us censored what we spoke about on social media or what we searched on the internet. We know from a 2013 PEN study that writers in the United States self-censored their browsing habits out of fear the government was watching. And this isn’t exclusively an American event; internet self-censorship is prevalent across the globe, China being a prime example.

It’s easy to imagine the more conservative among us getting enough power to make illegal what they would otherwise be forced to witness. In this way, privacy helps protect the rights of the minority from the tyranny of the majority.

Ultimately, this fear stagnates society in two ways. The first is that the presence of surveillance means society cannot experiment with new things without fear of reprisal, and that means those experiments—if found to be inoffensive or even essential to society—cannot slowly become commonplace, moral, and then legal. If surveillance nips that process in the bud, change never happens. All social progress—from ending slavery to fighting for women’s rights—began as ideas that were, quite literally, dangerous to assert. Yet without the ability to safely develop, discuss, and eventually act on those assertions, our society would not have been able to further its democratic values in the way that it has.

Consider the decades-long fight for gay rights around the world. Within our lifetimes we have made enormous strides to combat homophobia and increase acceptance of queer folks’ right to marry. Queer relationships slowly progressed from being viewed as immoral and illegal, to being viewed as somewhat moral and tolerated, to finally being accepted as moral and legal.

In the end it was the public nature of those activities that eventually slayed the bigoted beast, but the ability to act in private was essential in the beginning for the early experimentation, community building, and organizing.

Marijuana legalization is going through the same process: it’s currently sitting between somewhat moral, and—depending on the state or country in question—tolerated and legal. But, again, for this to have happened, someone decades ago had to try pot and realize that it wasn’t really harmful, either to themselves or to those around them. Then it had to become a counterculture, and finally a social and political movement. If pervasive surveillance meant that those early pot smokers would have been arrested for doing something illegal, the movement would have been squashed before inception. Of course the story is more complicated than that, but the ability for members of society to privately smoke weed was essential for putting it on the path to legalization.