1. Term

A sign in the most general sense is something understood to stand for something else, for something other than the sign itself. To serve as a sign, it must be recognized as signifying what it stands for. People and computer programs recognize and employ signs. To determine whether other animals do too depends on what counts as a sign, and on the assessment of their cognitive and instinctual functions. There is no unanimity as to what counts as a sign or how to classify different sorts of signs.

Some signs have a d…