Abstract: This paper examines evidence which can enhance our appreciation of the character and significance of Kant’s pre-Critical defence of physical influx and his conception of space-filling, in connection with his metaphysical concern for mind-body causal interaction. I highlight a Cartesian legacy in Kant’s True Estimation and his Physical Monadology, where Kant, modifying common conceptions of activity and spatial presence, distinguishes between the capacity of an immaterial substance to occupy space by having a spatial location, and the sphere of its activity, in contrast to the power of material substances to fill space by their extension and solidity. I highlight some important features of Descartes’s metaphysical and physical models of the contingent locality of simple unextended substances, and challenge recent views that Henry More’s model of extended but metaphysical-indivisible spirits is an archetype for, or at least a precursor to, Kant’s force-shell monads. I claim that contra More and the Newtonians, Kant appears indebted to Descartes for this idea of a simple substance’s external presence (in divisible space) in virtue of its (variable) power to act upon bodies or its sphere of activity, which does not affect its unextended nature or essence.
