Do I need documents when Test Driving my Development?

I was talking to a friend not so long ago about documentation. My argument is that writing external documents in order to define the behaviour and interactions of software is (or should be) covered by the business/stakeholders in their requirements or with the acceptance tests that have been defined for the application. There is of course a need to document the technical details, such as why you did something the way you did, and I feel this is best covered from self describing, well written, Unit Tests. More...