is severed to treat intractable epilepsy or due to a stroke. The standard neurological explanation is that the felt will reported by the speaking left hemisphere does not correspond with the actions performed by the non-speaking right hemisphere, thus suggesting that the two hemispheres may have independent senses of will. In addition, one of the most important ("first rank") diagnostic symptoms of schizophrenia is the patient's delusion of being controlled by an external force. People with schizophrenia will sometimes report that, although they are acting in the world, they do not recall initiating the particular actions they performed. This is sometimes likened to being a robot controlled by someone else. Although the neural mechanisms of schizophrenia are not yet clear, one influential hypothesis is that there is a breakdown in brain systems that compare motor commands with the feedback received from the body (known as proprioception), leading to attendant hallucinations and delusions of control. Experimental psychology Experimental psychology's contributions to the free will debate have come primarily through social psychologist Daniel Wegner's work on conscious will. In his book, The Illusion of Conscious Will, Wegner summarizes what he believes is empirical evidence supporting the view that human perception of conscious control is an illusion. Wegner summarizes some empirical evidence that may suggest that the perception of conscious control is open to modification (or even manipulation). Wegner observes that one event is inferred to have caused a second event when two requirements are met: The first event immediately precedes the second event, and The first event is consistent with having caused the second event. For example, if a person hears an explosion and sees a tree fall down that person is likely to infer that the explosion caused the tree to fall over. However, if the explosion occurs after the tree falls down (that is, the first requirement is not met), or rather than an explosion, the person hears the ring of a telephone (that is, the second requirement is not met), then that person is not likely to infer that either noise caused the tree to fall down. Wegner has applied this principle to the inferences people make about their own conscious will. People typically experience a thought that is consistent with a behavior, and then they observe themselves performing this behavior. As a result, people infer that their thoughts must have caused the observed behavior. However, Wegner has been