Occasionalism is a philosophical theory about causation which says that neither matter nor mind can be a true cause of events. Instead, all events are taken to be caused by God. The theory states that the constant conjunction between causes and effects only occurs because in every instance where the cause is present, God wills the effect to occur.
One of the motivations for the theory is the Dualist belief that mind and matter are fundamentally different things, and one cannot affect the other. Thus, a person's mind cannot be the true cause of his hand moving, nor can a physical wound be the true cause of mental anguish - in other words, the mental cannot cause the physical and vice versa. Still, Occasionalists generally also held that the physical cannot cause the physical either - apparently because of arguments similar to that of David Hume, who said that we cannot perceive any necessary connection between causes and effects. Thus, Occasionalism brings in God to fill this gap, since what God wills is taken to be necessary.
In a strict formulation of Occasionalism, the physical world never has any direct effect on anyone's perceptions. Considerations like this led George Berkeley to go beyond Occasionalism, and do away with the physical world entirely; Berkeley maintained that there was no such thing as matter, but merely perceptions, caused in us by God.
Nicolas Malebranche is well-known for his association with Occasionalism. One of the earliest philosophers to espouse a system of occasionalist beliefs was Abu Hamid Muhammad ibn Muhammad al-Ghazali, an 11th century Islamic writer.
Compare: Pre-established harmony
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