From Gödel, Escher, Bach, chap. 6, Douglas Hofstadter:
> In these examples of decipherment of out-of-context messages, we can separate out fairly clearly three levels of information: (1) the frame message; (2) the outer message; (3) the inner message. The one we are most familiar with is (3), the inner message; it is the message which is supposed to be transmitted: the emotional experiences in music, the phenotype in genetics, the royalty and rites of ancient civilizations in tablets, etc.
> To understand the inner message is to have extracted the meaning intended by the sender.
> The frame message is the message 'I am a message; decode me if you can!'; and it is implicitly conveyed by the gross structural aspects of any information-bearer.
> To understand the frame message is to recognize the need for a decoding-mechanism.
> If the frame message is recognized as such, then attention is switched to level (2), the outer message. This is information, implicitly carried by symbol-patterns and structures in the message, which tells how to decode the inner message.
> To understand the outer message is to build, or know how to build, the correct decoding mechanism for the inner message.
> This outer level is perforce an implicit message, in the sense that the sender cannot ensure that it will be understood. It would be a vain effort to send instructions which tell how to decode the outer message, for they would have to be part of the inner message, which can only be understood once the decoding mechanism has been found. For this reason, the outer message is necessarily a set of triggers, rather than a message which can be revealed by a known decoder.
> In these examples of decipherment of out-of-context messages, we can separate out fairly clearly three levels of information: (1) the frame message; (2) the outer message; (3) the inner message. The one we are most familiar with is (3), the inner message; it is the message which is supposed to be transmitted: the emotional experiences in music, the phenotype in genetics, the royalty and rites of ancient civilizations in tablets, etc.
> To understand the inner message is to have extracted the meaning intended by the sender.
> The frame message is the message 'I am a message; decode me if you can!'; and it is implicitly conveyed by the gross structural aspects of any information-bearer.
> To understand the frame message is to recognize the need for a decoding-mechanism.
> If the frame message is recognized as such, then attention is switched to level (2), the outer message. This is information, implicitly carried by symbol-patterns and structures in the message, which tells how to decode the inner message.
> To understand the outer message is to build, or know how to build, the correct decoding mechanism for the inner message.
> This outer level is perforce an implicit message, in the sense that the sender cannot ensure that it will be understood. It would be a vain effort to send instructions which tell how to decode the outer message, for they would have to be part of the inner message, which can only be understood once the decoding mechanism has been found. For this reason, the outer message is necessarily a set of triggers, rather than a message which can be revealed by a known decoder.