Understanding chemical reactivity has been the permanent concern of chemists from time immemorial. If we were able to understand it and express it quantitatively there would practically remain no unsolved mystery, and reactions would be fully predictable, with their products and rates and even side reactions. The beautiful developments of thermodynamics through the 19th century supplied us with the knowledge of the way a reactions progresses, and the statistical view initiated by Gibbs has progressively led to an unders­ tanding closer to the microscopic phenomena. But is was always evident to all that these advances still left our understanding of chemical reactivity far behind our empirical