Even though today's computers are used for many different types of tasks, they still rely on user interfaces designed for office workers in the 1980s. HCI researchers have produced a slew of innovative interaction styles, from gestural interaction to mixed reality and tangible interfaces, but they have not replaced traditional GUIs. I argue that we must devise fundamental principles of interaction that unify, rather than separate, these interaction styles in order to support the diversity of uses and users. I describe ongoing work on my ERC advanced grant, ONE, which explores how the concepts of information substrates and interaction instruments can help us create digital environments that users can appropriate and (re)combine at will.