Today I visited Amazon.co.uk. The website recognised me from the cookies it has left on my browser - there was no need to log in. At the top of the page are my recommendations: the Time Out Guide to Stockholm (clever, I want to visit Sweden again and Time Out is one of my favourite guidebook series); Free Culture by Lawrence Lessig (which I read several years ago); and an introduction to property valuation.
Perhaps, given the looming housing-market crash, this last recommendation was Amazon's little joke. More likely it stemmed from some books I bought six years ago when I was working for a property developer.
Amazon can recommend books to me because it knows what I have bought in the past. It also knows about the books I have browsed but not bought.
The web excels at this sort of automated tracking. But, as the purveyor of new web-tracking technology Phorm has found out over the past two months, that does not mean we're happy to have everything tracked.
Phorm's technology can dial direct in to your ISP's network and track your surfing habits as a user in order to deliver targeted ads. While Google serves up ads based on the page you are looking at now, Phorm will serve ads based on what you've looked at in the past. So if you've been looking up holidays lately, expect to see travel ads next time you're on your favourite news site.
BT, TalkTalk and Virgin are all signed up to trial Phorm's technology. But given the barrage of bad press it has received - culminating this month in virtual excommunication by the high priest of the worldwide web, Sir Tim Berners-Lee, they would be crazy to pursue this plan.
The public outcry over Phorm has been intense. What would a complete record of your surfing habits for the past week say about you? The data vapour trails we leave behind us as we navigate through the digital world are creating an ever more complete picture of our lives. BT, TalkTalk and Virgin obviously believed this picture belonged to them - that they were within their rights to use it to sell ad space. But the public backlash shows that somehow, fundamentally, we disagree with this analysis.
Over in Germany, the constitutional court has just delivered a landmark decision on data privacy that backs up this instinct. In essence, the decision accepts an individual's online behaviour as an "expression of personality", an activity whose integrity and confidentiality are fundamentally protected in German constitutional law.
The more sophisticated the tracking and analysis of our behaviour in the digital world becomes, the greater the detail about our lives that commercial and state bodies could have access to. The outcry over Phorm suggests it is time for a serious debate about who can gain access to our data in the digital age.