The aim of this chapter is to prepare for a discussion of medial difference by first focusing on what all media share, outlining a series of categories that will be useful in discussing all media. In the following chapters, these categories are represented differentially once we begin comparing kinds of media. The first observation is that, ontologically, medial objects are instances of cultural objects generally. They are all purely intentional objects in Ingarden's sense. In light of the variety of media – visual, aural, spatial, temporal – it is affirmed that space is ontogenetically prior in media. In an important essay, Arnheim introduced two kinds of images that can be applied to all media – the robin and the saint, the former a "likeness' and the latter a 'self-image." They coexist in any image or medial object and one represents the thing as self-sufficient and the other as referring externally. The way in which a sculpture of a naturalistic robin or an abstracted St. Francis in a garden have these two functions in different measure suggest differences in classes of media.