In Physics it has been shown that a body in acting upon another body must either directly be in contact with it, or indirectly through the medium of other bodies. E.g., a body that has been heated has been in contact with fire, or the air that surrounds the body has been heated by the fire, and has communicated the heat to the body; the immediate cause of the heat in this body is the corporeal substance of the heated air. The magnet attracts iron from a distance through a certain force communicated to the air round the iron. The magnet does therefore not act at all distances, just as fire does not act at every distance, but only as long as the air between the fire and the object is affected by the fire. When the air is no longer affected by the fire which is under a piece of wax, the latter does not melt. The same is the case with magnetism. When an object that has previously not been warm has now become warm, the cause of its heat must now have been created; either some fire has been produced, or the distance of the fire from the object has been changed, and the altered relation between the fire and the object is the cause now created. In a similar manner we find the causes of all changes in the Universe to be changes in the combination of the elements that act upon each other when one body approaches another or separates from it. There are, however, changes which are not connected with the combination of the elements, but concern only the forms of the things; they require likewise an efficient cause; there must exist a force that produces the various forms. This cause is incorporeal, for that which produces form must itself be abstract form, as has been shown in its proper place. I have also indicated the proof of this theorem in previous chapters. The following may, in addition, serve to illustrate it: All combinations of the elements are subject to increase and decrease, and this change takes place gradually. It is different with forms; they do not change gradually, and are therefore without motion; they appear and disappear instantaneously, and are consequently not the result of the combination of corporeal elements. This combination merely prepares matter for receiving a certain form. The efficient cause which produces the form is indivisible, because it is of the same kind as the thing produced. Hence it may be concluded that the agent that has produced a certain form, or given it to a certain substance, must itself be an abstract form. The action of this incorporeal agent cannot depend on a certain relation to the corporeal product; being incorporeal, it cannot approach a body, or recede from it; nor can a body approach the incorporeal agent, or recede from it, because there is no relation of distance between corporeal and incorporeal beings. The reason why the action has not taken place before must be sought in the circumstance that the substance has not been prepared for the action of the abstract form.