Interchangeability Refers to the idea that humans can give and receive identical linguistic signals; humans are not limited in the types of messages they can say/hear. One can say "I am a boy" even if one is a girl. This is not to be confused with lying (prevarication). The importance is that a speaker can physically create any and all messages regardless of their truth or relation to the speaker. In other words, anything that one can hear, one can also say.