In March, exactly twenty-five years after he first outlined his proposal for a "world-wide web", Tim Berners-Lee called for "a global constitution - a bill of rights" to protect the "neutral, open internet". This appeal for a "Magna Carta", a single document that enshrines certain fundamental rights and protections for citizens in the digital age, comes after a series of revelations about the extent of state surveillance online.

And this state surveillance dovetails the business models of Facebook, Google and the other tech giants. Their profits derive from the ability to hoover up data from users of their "free" services and sell it to other businesses. They can charge a premium for a vision of the world where the wasteful banging on a swill bucket that is mass media advertising has been replaced with millions of moments of bespoke manipulation. We are living through the enclosure of subjectivity, one voluntary disclosure at a time.

What internet freedom?

This combination of state and corporate insinuation in the daily lives of citizens poses a serious threat to the liberty of the individual. But it is important to understand what we mean by liberty in this context. In mainstream liberalism, liberty is the absence of interference. As long as we are free to go about our lawful business without running into arbitrary restrictions we are free. It is this notion of freedom that politicians draw on when they assure us that intelligence agencies operate in a framework of regulation and oversight and that the law-abiding citizen has nothing to fear from the NSA or GCHQ.

But there is another, older and more demanding, conception of freedom. It derives from Roman law and distinguishes between the liber homo, the free man, and the servus, the slave. Quentin Skinner, the political theorist who has done most to rediscover and popularise this pre-liberal liberty, explains that "a slave is someone who is in potestate, in the power of a master". A free man, on the other hand, "is sui iuris, able to act in their own right".

Crucially, a free man is not someone who happens to be left alone in the pursuit of his goals. A slave who has a tolerant or easily duped master can enjoy that kind of license. But a free man doesn't depend on anyone else for his freedom. He is free in virtue of his status as a citizen living under a republican constitution.

The freedom espoused by liberals has no particular implications for the form of government. As long as we are left alone in our lawful pursuits we are free, whether or not we have the vote. But neo-Roman, or republican, liberty is irreducibly political. It rests on the powers that the citizen can use against both state and private domination. It isn't enough that we are left alone. We must be confident that no one can interfere with our individual and collective plans.

Interference can take many forms and the republican imagination is much occupied with exploring them. We can be subject to outright coercion, we can be induced to act in certain ways through economic incentives and threats, and we can be manipulated so that we think we are acting freely but are in fact serving someone else's agenda. Most people in Anglo-America are not exposed to the first form of interference, though low income groups tend to have much more experience of it than their more affluent compatriots are comfortable admitting.

Economic domination is much more pervasive. Many of us spend our days doing things we wouldn't do if we didn't need to make a living. And we are also vulnerable to manipulation. It is hard to say how much manipulation goes on. Conventional liberals have tended to be dismissive of the danger. But this seems increasingly untenable in light of recent history.

In the early years of this century we were all making choices on the basis of radically unsafe information, as the financial crisis revealed. Besides, if you think that you can spend your working life doing things you'd rather not do and still be free, you might want to ask how you came to that conclusion.

Vulnerability to manipulation

The republican conception of freedom is particularly relevant when we turn to the internet. For here the problem is not primarily one of overt coercion. Rather, as digital spaces become the primary means by which we share and discuss publicly relevant information, we become vulnerable to manipulation by those who own and infiltrate those spaces. Remember, this is implicit in the business model of the tech giants.

The state is making extensive use of corporate data-gathering. It could also use the corporations as instruments of extensive manipulation. Indeed, thanks to Edward Snowden, we know that it is making plans to do so. The manipulation of civil society by the state is best understood as a form of covert domination. We are free to do what we want, but the state tailors information to ensure that what we want to do doesn't get in the way of their plans. The barbed wire isn't around us. It is in our heads.

So state and corporate attempts to subvert an open and neutral internet should be taken very seriously. If we want to be substantially free we need to engage with the campaign for the Web We Want. But if we are looking for inspiration from medieval English charters, we would benefit greatly from taking another look at Magna Carta's less famous companion, Carta de Foresta, the Charter of the Forests. Issued in 1217 it prevented the King from designating land as "forest" and granted important rights of access and use to ordinary people. This early victory against enclosure ensured that royal lands were not the exclusive preserve of the powerful but a common resource, available to those who needed them.

At the moment the modern-day heirs of the English Kings, GCHQ, Facebook and the rest, are busy enclosing the common possession that is the internet. Indeed they are going further. Their ambition is to re-order the wilderness of our individual wants and world-views into an orderly estate, in which broad avenues of conventional opinions stretch to the horizon.

In order to prevent them we will need new powers. Powers to initiate and fund inquiry, powers to address our fellow citizens, powers to oversee the operations of the state and their partners in the corporate sector become the modern equivalents of the ancient rights to graze pigs and gather firewood in the forest and - pannage and estover, according to Wikipedia, in case you were wondering.

For if we are to guard against manipulation, we will need to be able to challenge and effectively refute those who are currently in the business of regulating global public opinion. Where they profit from the internet as a collection of private properties and secret resources we can only secure our freedom by making the internet into a res publica, a collective possession. The fight for the internet is central to the struggle for public liberty.

Dan Hind is the author of two books, The Threat to Reason and The Return of the Public. His e-book, Maximum Republic is published later this month.