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'. Yet, experimentalists tell us quantum theory is the most successful theory in history. Quantum theory cannot be understood as a classical mechanical theory since it arose through the 'interpolation' of two highly successful but complementary classical mechanics: Newtonian particle mechanics and Maxwellian wave mechanics. The two-slit experiment illustrates that what is experienced depends on choice of experimental set-up. Quantum theory is properly understood within the more general framework of engineering thermodynamics. In Part One, I point to four essential characteristics of quantum theory that cannot be understood in any framework defined by the classical mechanical presuppositions of symmetry and conservation. These four characteristics are the participatory, the complementary, the indeterminate and the new non-commutative geometry. 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. Leibniz brought these forth in his meta-paradigm shift from Statics to Dynamics. This article is part of the theme issue 'Thermodynamics 2.0: Bridging the natural and social sciences (Part 2)'.