Occasionalism is a philosophical theory about causation which says that created substances cannot be efficient causes of events. Instead, all events are taken to be caused directly by God. (A related theory, which has been called 'occasional causation', also denies a link of efficient causation between mundane events, but may differ as to the identity of the true cause that replaces them.) The theory states that the illusion of efficient causation between mundane events arises out of God's causing of one event after another. However, there is no necessary connection between the two: it is not that the first event causes God to cause the second event: rather, God first causes one and then causes the other.