Interior design is a neglected practice within the broader framework of design theory. Indeed, it is often misunderstood by the general public and sometimes regarded problematically among better-recognized design practices such as graphic design, industrial design, interaction design, and service design. However, with careful attention to the central themes of interior design, one may gain a new perspective on the nature of design itself and the unfolding development of design thinking from the late nineteenth century to the twenty-first century. Foremost among these themes is the transformation of the surrounding physical conditions of our lives into environments for human action and experience. This theme is explored through the creative matrix of the four orders of design, reflecting a shifting focus from place, space, and action to interiors of the mind. Interior design is an interesting and perhaps surprising example of fourth order design, where dialectical humanism plays a central role as an organizing theme for the great diversity of fourth order projects in the twenty-first century.