The previous chapter depicts mathematics as a collective enterprise: Ptolemaic officers were told to check the accounts in groups; the surveyors at Heraclea measured the land together; the central element of a catapult was identified thanks to accumulated experience. The prefaces to several mathematical texts recount of networks of people, who sometimes communicated by letter, sometimes physically met and talked to each other, maybe pored over a diagram together. Some other times, they communed across the boundaries of time, in a dialogue with past works – take Archimedes and Eudoxus, or Apollonius and Euclid. One of the two questions raised in this chapter will be about communities of mathematicians. I will try better to describe them, and to relate them to the wider context of Hellenistic culture. But before I do that, I will tackle another question.