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. 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). However, an ability is “a power to originate changes in the environment.”
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 can understand French has no choice over whether or not to understand French sentences when he hears them. In contrast, his ability to speak French does not entail that he utter a word in French. Sentences in French uttered or inscribed cause a person with the capacity to understand French to grasp the correct meanings of the sentences. But a person who can speak French is not caused to do so merely by having such an ability.
Peter Van Inwagen, An Essay on Free Will (New York: Oxford University Press, 1983), 10–11.