1. IntroductionThe will is an executive process involving the formation and translation of mental states into choices and actions. Some philosophers have defined the will as the mental act of trying.1 Neuroscientists tend to define it as neurobiologically based volition associated with unconscious brain processes underlying basic actions.2 Yet it seems more plausible to define the will as a cluster of conative, cognitive, affective, and physical capacities extending from an initial desire to act to the performance of the action. These capacities collectively provide an agent with control of her thought and behavior.The mental capacities necessary for self-control are mediated by neuronal processes involving re-entrant loops consisting of the subcortical basal ganglia and thalamus, limbic structures such as the amygdala, and prefrontal, anterior cingulate, and parietal cortices.3 Component structures of the basal ganglia, such as the striatum, primarily regulate motor control but also some of the conative, cognitive, and affective processes necessary for motivation, attention, and planning. The thalamus relays information from the sensory cortex to the amygdala and higher-brain regions. It also plays a critical role in conscious arousal and awareness through its projections to the cortex. The amygdala mediates emotion, and the anterior cingulate is crucial for empathy and the perception and resolution of conflict. Prefrontal areas of the cerebral cortex mediate practical and moral reasoning. Interacting cortical, limbic, and subcortical pathways provide the neurobiological underpinning of the mental and physical capacities necessary to reason, decide, and act. Still, the desires, beliefs, emotions, and intentions that constitute the will are not reducible to physical processes in the brain because these processes cannot capture the phenomenology, or subjective experience, of having these mental states in ordered or disordered forms.All persons with the capacity for conscious awareness have a will. But they possess and exercise it to varying degrees. The extent to which the will is free depends on whether the neurobiological processes that sustain the requisite mental and physical capacities for deliberation, choice, and action are functional or dysfunctional. This can be a matter of degree as well. Some of these capacities may be intact, while one or more may be impaired, which can diminish the will by constraining or compelling action. A person with Parkinson's disease may retain conative, cognitive, and affective capacities necessary to form plans of action but have diminished physical capacity to execute these plans because of impaired motor control. One with schizophrenia who experiences delusions may be sufficiently motivated and physically able to act but impaired in the cognitive capacity to have appropriate beliefs and respond to reasons for or against actions. A person with major depression may retain cognitive and physical capacities, but anhedonia (inability to experience pleasure from previously pleasurable activities), avolition (lack of motivation to pursue meaningful goals), and low mood may impair his conative and affective capacity to motivate himself to act. Even when there is no neurological or psychiatric abnormality, we do not always completely control our behavior because of unconscious neural processes involved in the performance of our actions.Drugs such as dopamine agonists or deep-brain stimulation (DBS) may restore motor control in Parkinson's disease by compensating for depletion of dopaminergic neurons in the substantia nigra region of the basal ganglia. DBS for this disease involves implanting and stimulating electrodes in this same brain region to modulate overactive or underactive neural circuits regulating movement. Antipsychotic medication may resolve the delusions and restore the cognitive functions regulating the schizophrenic's beliefs to normal or near-normal levels. …