(1) The “mechanical philosophers” of the early seventeenth century held that any material body maintains a constant velocity unless acted on, and moreover held that all action is due to impact between one material particle and another. So stated, the mechanical philosophy immediately precludes anything except impacting material particles from producing physical effects. Leibniz saw this clearly, and concluded that it discredited Descartes’ interactive dualism, which had a non-material mind influencing the physical world (Woolhouse 1985). (Of course, Leibniz did not therewith reject dualism and embrace the physicalist view that minds are composed of material particles, but instead opted for “pre-established harmony”.  Views which avoid ontological naturalistic views of the mind by denying that mental events have any physical effects will be discussed further in section 1.6 below.)