The project of a science studying all possible varieties of signs and the rules governing their production, exchange, and interpretation is a rather ancient one. Pre-Socratic poetry and philosophy are frequently concerned with the nature of natural signs and divine messages. The Hippocratic tradition deals with the interpretation of symptoms, while the Sophists were critically conscious of the power of language. Plato's Cratylus is a treatise on the origins of words, and the Sophist can be considered the first attempt to apply a binary method to semantic definitions.