second draft of this book
of this book
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Structure describes how components are put together to make a complex entity. Essence is the inherent fundamental nature of an entity. In traditional analysis the essence of a complex structure is determined by the essence of its components. For example a steel bridge is strong because steel is strong. It can withstand high winds because steel is flexible. At one time it was thought that everything was made of earth, air, fire and water. The innate properties of these primary elements determined the essential nature or essence of everything else.