Design for content, not devices or screen widths. Whilst employing responsive design principles, we must build to content first, and devices second. We should aim for content parity across media and devices.
Design exists to facilitate provision of information and services; to enhance that interaction and engagement.
Content and services exist to meet user needs and deliver business goals.
The web is a self-service medium. We're designing to facilitate people doing what they need to do independently.
So we're not designing primarily for devices or screen widths, Fand our aim is parity across media and devices.