The field of ubiquitous computing was inspired by Mark Weiser's vision of computing artifacts that disappear. "They weave themselves into the fabric of everyday life until they are indistinguishable from it." Although Weiser cautioned that achieving the vision of ubiquitous computing would require a new way of thinking about computers, that takes into account the natural human environment, to date no one has articulated this new way of thinking. Here, we address this gap, making the argument that ubiquitous computing artifacts need to be physically and cognitively available. We show what this means in practice, translating our conceptual findings into principles for design. Examples and a specific application scenario show how ubiquitous computing that depends on these principles is both physically and cognitively available, seamlessly supporting living.