Can robots communicate, and if so, in which sense? What about apes, ants, or neurons?
On the basis of several examples, I shall analyze the essential aspects of communication, from a simple transfer of information that is beneficial to the sender to a recognition of communicative intentions. This raises issues of contingencies, that is, degrees of freedom of the sender and the receiver, and their resolution through conventions of symbol use, at the level of the individual agents as well as at an evolutionary one. I shall also sketch a mathematical framework for formalizing and quantifying these aspects.