The reason why people put movement in these classes is that it is thought to be something indefinite, and the principles in one of the two 'columns of contraries' are indefinite because they are privative, for none of them is either a 'this' or a 'such' or in any of the other categories.
izan ere, hauetako bat bera ere ez da ez "hau", ez "horrelakoa", ezta bestelako kategoriaren bat ere.