But it is not every activity of the passionate part of the soul that is called passion, but only the more violent ones, and such as are capable of causing sensation: for the minor and unperceived movements are certainly not passions. For to constitute passion there is necessary a considerable degree of force, and thus it is on this account that we add to the definition of passion that it is a sensible activity. For the lesser activities escape the notice of the senses, and do not cause passion.