A person entering a room experiences that event bodily: through movement, orientation, intention, proximity, hesitation and awareness of others. A sensor does not encounter these qualities as such. In the working prototype, two visible laser beams positioned across the entrance detect the sequence in which the beams are interrupted. From this sequence, the system infers whether someone has entered or left and adjusts an occupancy count. An ultrasonic sensor provides an additional indication of whether someone remains inside. The system does not first encounter a complete bodily event and subsequently reduce it. What becomes available to the system is already conditioned by the apparatus through which the encounter takes place.

The sensor does not receive a person and remove their intention, social relationship or bodily experience. It encounters an interruption. The interruption becomes a crossing. The crossing becomes a count.

Realistically to make it possible, I imagined a room surrounded by sheer curtains. More people enter and the curtain walls move inward. People leave and they move outward again. The curtain mattered.

We already use curtains to negotiate our relationship with other people. I close one when I don't want to be seen. I open it when I want the outside back in. Sometimes I leave it halfway. A curtain sits somewhere between inside and outside, privacy and exposure. It does not make the world disappear. It lets me decide something about my relationship to it.

Personal space is not really an invisible wall around me. It changes. I let somebody come close. I move away from somebody else. I make myself available and later I withdraw. A curtain already knows that gesture. And unlike a wall, it can move. It folds. It gathers. Light passes through. A silhouette remains visible without becoming completely exposed. I think of the curtain as a membrane.