This Friday I will be giving a keynote for the Media, Communication, and Cultural Studies Association of the UK (MeCCSA) exploring the question of what a medium is beyond the discussion in Inventing the Medium. 
I will be talking about the four existing models of Media Theory, and about the new model I discuss in ITM and in an earlier article for Popular Communication, which is based on the work of Merlin Donald and Michael Tomasello.
This Hellenistic period terracotta of two women playing the ancient game of knucklebones – a form of dice – (from an image on the British Museum website) is iconic for me of one way to think about what a medium is.
Janet H. Murray 