A capacity is kind of power, but it is not the same as an ability. An ability is a power that an agent has to perform some action. It is “a power to originate changes in the environment.” In contrast, a capacity is “a causal power…an invariable disposition to react to certain determinate changes in the environment in certain determinate ways.” (Van Inwagen, An Essay on Free Will, 10-11). For example, Van Inwagen distinguishes between the capacity to understand and the ability to speak. The distinction lies in the fact that though someone who has the capacity to understand French has no choice over whether or not to understand French sentences…