By spitting in Sienna Miller's face in order to create a suitably sensational image, the paparazzi remind us of an important point. Nothing in human life is inherently private. Certainly not urinating, defecating and copulating, which only a few centuries ago could be performed in public with no sense of shame. Bedrooms were not particularly private places in medieval Europe, and wanting to relieve yourself unobserved might be considered as eccentric as wanting to crack jokes in utter solitude.

The frontier between public and private shifts from time to time and culture to culture. In pre-modern Europe, the three great spiritual or symbolic areas of existence – religion, sexuality and art – were all public affairs. Religion was a mighty political force, in the alliance between throne and altar. Sexuality was a question of dynastic marriage, the union of landed estates and the generation of labour power. The artist was the paid hireling of church, court or state.

All this meant that art, religion and sexuality really counted. They carried vital weight in the public sphere. In fact they carried so much weight that you could be burned at the stake for heresy, have your door broken down by the purity police, or be instructed by the ruling powers what to write, paint or compose. With the advent of modernity, however, some much-needed privacy set in. The privatisation of God was a case in point. Religion for Protestantism became an issue between you and the Almighty. It was no particular concern of the state. As for sexuality, you could now hook up with whoever took your fancy, as the concept of romantic love began to take root.

Yet there was a paradox here. Privatising art, religion and sexuality set them free, but it also sailed dangerously close to trivialising them. Like breeding gerbils or collecting toby jugs, they were now nobody's business but your own. If nobody was burned at the stake any longer, it was partly because spiritual values had less and less force in a materialist society. Few people complained about outrageous experiments in art, since art seemed pretty pointless in any case. If you could leap into bed with whoever you liked, it was partly because sexuality had been stripped of its social and political dimensions. The word "private" is related to "privation", suggesting that whatever is withdrawn from the public realm has no real existence. "Private" meant "hands off", but it also meant "of no great importance". So if the person you slept with was of no great importance, what was wrong with having it broadcast to the world?

The modern age has achieved some balance in this respect. We cherish sexual freedom and privacy while believing with our pre-modern ancestors that sexuality is a public, political affair. The state should not forbid contraception, but that is not to say that it should turn a blind eye to domestic violence.

There is one respect, however, in which the balance between public and private is grotesquely awry. It is a feature of capitalism that the public world of commerce, finance, trade and manufacture falls under the sway of private interests. And this includes the media. So the publicising of the private, as in the hounding of Sienna Miller, is a result of the privatising of the public.

In the end, it is because the media are driven by the power and wealth of private individuals that they turn private lives into public spectacles. If every private life is now potentially public property, it is because private property has undermined public responsibility. It is our respect for human privacy which makes phone hacking so repugnant, and our respect for private profit which allows it to flourish. This is a contradiction it will take more than a parliamentary committee to resolve.

• For legal reasons this article will not be open to comments