  In such fertile circumstances – which included tutoring by  Huygens himself and direct access to Descartes’s unpublished  notebooks – Leibniz quickly fashioned his own penetrating  critique of Cartesian physics. As early as   1676,[7]  he had found what he considered to be a fatal flaw in Descartes’s  cornerstone conservation law, namely, that it violates the principle  of the equality of cause and effect. When Leibniz published  essentially the same objection in his Brief Demonstration of a  Notable Error of Descartes’s and Others Concerning a Natural Law in 1686, it sparked a now famous dispute among natural philosophers  that has become known as the vis viva controversy  (GM VI 117–119/L   296–298).[8]  For Leibniz the argument of the Brief Demonstration marked  not only an important event in his understanding of the Cartesian  system, but also a turning point in the development of his own  distinctive physics. 