  The heterogeneity of matter in Holbach’s account contributes to the vagueness of that designation. Matter can to some extent be understood in the ordinary sense of anything that has extention, figure, and so on. However, because matter also may or may not have any number of properties not ordinarily understood to belong to matter, such as thought, it is not entirely clear what may not be matter. Motion is likewise and for similar reasons a vague term in Holbach. Where matter is understood simply as extension and some other very simple properties, motion may be thought of in similarly simple terms, as a velocity, acceleration or, perhaps, as an impulse with a certain direction. Once matter is thought of, after the manner of Holbach, as something with properties which are perhaps best not understood in spatial terms, its motion may be much more difficult to define. Although he sometimes speaks of matter and motion in narrower senses, Holbach’s tendency is simply to identify matter and motion with the general terms cause and effect. Holbach typically identifies bodies with causes and motions with effects, but he also allows that motions may be causes: