Nicolas Malebranche denied that anything, either mental or physical, could cause, or be the effect of, anything else. His reasoning was that physical bodies were inert and passive, without any force within them that could cause anything or even sustain movement. Neither can mental things cause anything, because there is no necessary connection between any human act of will and any other event. Only God has an effective will in this sense. Therefore, all causal connections in nature are in reality the actions of God. Causal chains in nature are like two clocks that are one minute apart in time. There may be an appearance of the clock that is ahead in time causing the movements of the slower clock, but this is no more than an appearance.