At a time when it is argued that we have arrived at a post-medium condition, German artist Gerhard Richter demonstrates that questions of medium remain vitally relevant to art today. In particular, his paintings and their explicit relation to photography make clear that media define each other and that this is essential to their use and function. That is, we can only ever know painting through photography or photography through painting. This is what Rosalind Krauss calls the 'differential specificity' of media and what Michel Foucault calls 'disguised difference'. Richter makes these differences and their signifying potential visible and this enables us to know far more of the role of medium in art today. More particularly, the success of Richter's sustained critical attention to mediality, as enacted through this dialogue between photography and painting, is based in what Jacques Derrida describes as the function of idiom. It is idiom that enables the differences between media that are crucial to their signification and upon which the medium exchange that is necessary to art is based.