Quantum theory only makes sense in Lazare Carnot's participatory engineering thermodynamics, a development of Leibniz's dynamics.Feynman insisted 'no one understands quantum theory'.  Quantum theory is properly understood within the more general framework of engineering thermodynamics.  In Part Two, articulating engineering thermodynamics, I note there are two histories and two formulations of thermodynamics: Carnot's engineering thermodynamics and the 'rational mechanical' tradition of Clausius-Boltzmann.  These four essential characteristics of quantum theory are also characteristics of engineering thermodynamics.  In Part Three, I trace the precursors of Lazare Carnot's engineering thermodynamics to earlier insights of Huygens, d'Alembert, Leibniz and the Bernoullis.  This article is part of the theme issue 'Thermodynamics 2.0: Bridging the natural and social sciences (Part 2)'.
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