In 1948, the promised memorandum appeared as "A Mathematical Theory of Communication", an article in two parts in the July and October issues of the "Bell System Technical Journal". This work focuses on the problem of how best to encode the information a sender wants to transmit. In this fundamental work, he used tools in probability theory, developed by Norbert Wiener, which were in their nascent stages of being applied to communication theory at that time. Shannon developed information entropy as a measure of the information content in a message, which is a measure of uncertainty reduced by the message, while essentially inventing the field of information theory.