  Bk. 8 of the Physics argues for the additional thesis that for each motion, whether natural or contrary to nature, there needs to exist a mover. [42]  In cases of forced motion, movers are present in a perspicuous way. This need not be so, however, in cases of natural motion.  Apart from the cases where the nature of the entity is at the same time a moving and efficient cause—i.e., apart from living beings, whose nature, the soul, is both formal and efficient cause—the mover may be inconspicuous. This is eminently so in the remaining large class of natural motions, the natural motions of the elements. The nature of these elements, their inner principle of motion and rest is not the moving cause of the motions of the elements, Aristotle claims. If it were, then it would be up to the elementary masses to determine when they should perform their motions, but plainly it is not. Moreover, the principle of causational synonymy rules out that any homogenous mass, without an internal demarcation into components which move and are moved, could move itself (Physics 8.4, 255a5–18). This is so because, on the assumption that one part of a homogenenous body could move another part, the active component of change would be, in every aspect, indistinguishable from the part in which change is effected, and this in turn would mean that change would occur even though there would be no transmission of a causally relevant property from the active part to the passive. This implies that even though we may answer the question as to why the elements move to their natural places—the light bodies up and the heavy ones down—by an appeal to their respective natures as causes (“that it is simply their nature to move somewhere,[43]  and this is what it is to be light and to be heavy,” Physics 8.4, 255b13–17), we do not thereby specify their moving causes. Their thrust being in a single direction, the elements cannot circumvent even rather simple obstacles they may encounter on their way (a sealed container can retain air under water, the roof stays put pressing down on the walls of a building etc.). Hence, whoever removes an obstacle to an element’s motion is causally responsible for the ensuing elemental motions. But even such a causally responsible agent will not qualify as the moving cause, without yet further qualifications.  For the identification of the moving cause of these locomotions Aristotle invokes his distinction of two potentialities.  Some heavy material can be potentially light, as it can be transformed into a light material in a process of generation, whereas the emerging light material is still potential in a sense until it has acquired its full-fledged status, which involves its having arrived at that region of the cosmos which is its natural place.  This analysis, then, describes the natural locomotion of the elements as a possibly postponed, completing stage within a single overarching process, and hence in these cases Aristotle can identify the cause of the second stage of the process with the efficient cause of the first stage, the entity which generated the element in the first place (Physics 8.4, 256a1).