Eric Watkins argues that a grasp of Leibnizian and anti-Leibnizian thought in eighteenth-century Germany helps one to see how Kant (in his critical period) argued for causal principles that have both metaphysical and epistemological elements. According to this interpretation, Kant's model of causality does not consist of events, but rather of substances endowed with causal powers that are exercised according to their natures and circumstances.