Investigations into semiotic theory typically begin with the fundamental question: what is a sign? A definition is then offered, usually quoted from an established authority (such as C. S. Peirce), to get the argument started. But an a priori definition immediately begs a basic methodological question: how does the author of such a definition know that he or she is right? How did C. S. Peirce for example know that a sign is what his celebrated definition (Peirce 1998: 135) says it is. He may well have been right but he gives us no proof of that. His and similar ex cathedra definitions used in semiotics are often little more than intellectual opinions and intuitions presented to the reader to be accepted on faith, but they are not logical conclusions deriving from clearly stated premises. So how else can we arrive at a logically valid and possibly useful understanding of signs?