To see how Shannon’s choice embodies a self-reflexive moment, it will be necessary to understand more precisely how informational entropy is like and unlike thermodynamic entropy. Shannon defined information as a function of the probability distribution of the mes-sage elements.17 Information in Shannon’s sense does not exist in the same way as the dimensions of this book exist. A book can be mea-sured as twelve inches long, even if there are no other books in the world. But the probability that a book has that dimension is mean-ingful only if there are other books with which it can be compared. If all books are twelve inches long, the probability that a given book has that dimension is i, indicating complete certainty about the re-sult. If half of the books are twelve inches, the probability is 1/2; if none are, it is o. Similarly, information cannot be calculated for a message in isolation. It has meaning only with respect to an ensem-ble of possible messages.