and to become voluntary afterwards by degrees. However, the Stahlians agree with me concerning the near relation of these two sorts of motion to each other, as also concerning the transition (or rather return, according to my hypothesis) of voluntary motions into involuntary ones, or into those which I call secondarily automatic. As to final causes, which are the chief subject of inquiry amongst the Stahlians, they are, without doubt, every where consulted, in the structure and functions of the parts; they are also of great use for discovering the efficient ones. But then they ought not to be put in the place of the efficient ones; nor should the search after the efficient be banished from the study of physic, since the power of the physician, such as it is, extends to these alone. Not to mention, that the knowledge of the efficient causes is equally useful for discovering the final, as may appear from many parts of these observations.
Cor. III. It may afford the reader some entertainment, to compare my hypothesis with what Des Cartes and Leibnitz have advanced, concerning animal motion, and the connexion between the soul and body. My general plan bears a near relation to theirs. And it seems not improbable to me, that Des Cartes might have had success in the execution of his, as proposed in the beginning of his Treatise on Man, had he been furnished with a proper assemblage of facts from anatomy, physiology, pathology, and philosophy, in general. Both Leibnitz’s preestablished harmony, and Malebranche’s system of occasional causes, are free from that great difficulty of supposing, according to the scholastic system, that the soul, an immaterial substance, exerts and receives a real physical influence upon and from the body, a material substance. And the reader may observe, that the hypothesis here proposed stands clear also of this difficulty. If he admit the simple case of the connexion between the soul and body, in respect of sensation, as it is laid down in the first proposition, and only suppose, that there is a change made in the medullary substance, proportional and correspondent to every change in the sensations, the doctrine of vibrations, as here delivered, undertakes to account for all the rest, the origin of our ideas and motions, and the manner in which both the sensations and these are performed.
Cor. IV. I will here add Sir Isaac Newton’s words, concerning sensation and voluntary motion, as they occur at the end of his Principia, both because they first led me into this hypothesis, and because they flow from it as a corollary. He affirms then, “both that all sensation is performed, and also the limbs of animals moved in a voluntary manner, by the power and actions of a certain very subtle spirit, i.e. by the vibrations of this spirit, propagated through the solid capillaments of the nerves from the external organs of the senses to the brain, and from the brain into the muscles.”
