For the past two years weve been using a personal sensing system we call SPEC. Because it has different goals it has a different design philosophy, and thus its architecture is quite different from the more common environmental sensing system. It was designed to monitor a users activities for their own purposes, in order to support everyday information processing activities. In other words, it is not a surveillance system. It uses collections of personal, private sensors, called SPECs, which can be worn, or deployed where each individual user needs them. This P2P system is designed on the principle that for widespread adoption the users must feel confident that whatever information they sense about themselves is kept under their own control, and not some central agency-however apparently benign. This places it in sharp distinction to RFID technology for example. After two years experience building applications with our system, we are convinced that this approach has a lot to offer, and we are embarking on a program to build an improved version, and take this opportunity to review the consequence of our design philosophy, and identify areas for further research. 14 Pages