When MIT grad student Steve Mann began wearing a computer and a head-mounted camera every moment of the day in 1981, he wasn’t thinking of Black Lives Matter, racism, or police violence. But Mann, now a professor at the University of Toronto, may have given us the key concept for understanding the role of ubiquitous cameras in documenting police violence against people of color.

The recent shootings of Alton Sterling in Baton Rouge, Louisiana, and Philando Castille in Falcon Heights, Minnesota, were documented on multiple cameras. Sterling’s death outside a convenience store, where he worked selling CDs, was captured on the store’s surveillance camera and by two bystanders on their cell-phone cameras. Castille’s death may have been captured by a squad car camera when he was stopped for a broken taillight. But we know about his death because Diamond Reynolds, Castille’s fianc