 The second source of resistance to compatibilism stems from confusion about necessity. People are prone to think that necessity rules out liberty because they conflate necessity with compulsion or force. They believe that when A causes B, A compels or forces B. Hume holds that this is a mistake, however. Causal necessity is nothing more than the “constant conjunction” of similar objects and a customary inference of the mind from the one to the other (EHU 8.1.5). For example, experience shows that the vibration of a particular string is constantly conjoined with a particular sound. As a result, the mind develops the habit of immediately inferring the sound from the vibration (EHU 7.2.29). We believe that the sound is causally necessitated by the vibration, but the necessity is a product of the imagination, which associates the idea of the sound with the perception of the vibration. This feeling that there is a necessary connection between them is the whole of the matter (EHU 7.2.28). So defined, causal necessity no longer poses an obvious threat to liberty. It makes sense to follow Bramhall’s definition of liberty as “liberty from necessity” if one thinks that necessitation involves force or compulsion. But causes do not force or compel their effects. If one can accept this point, then a definition like Bramhall’s seems less tempting, and necessity per se no longer poses a threat to liberty. The only real threat is necessitation by external forces, as when a person is forced to do something she does not want to do.