 The initial wave of philosophical work done on diagrammatic reasoning focused on Euclid’s Elements, partly because of the centrality and historical importance of this work, and partly because it is so often held up as a canonical example of the deductive method (see, e.g., Mumma 2010). If some or all of the diagrams in the Elements fall under option (iii) above, then deleting all the diagrams will render many of the proofs invalid. This raises the further question of whether a distinctively diagrammatic form of reasoning can be identified and analyzed, and—if so—whether it can be captured in a purely deductive system. One difficulty for any proposed rigorization is the ‘generalization problem’: how can a proof that is linked to a specific diagram be generalized to other cases? This is intertwined with the issue of distinguishing, in formal terms, between the essential and coincidental features of a given diagram. 