In the ancient philosophy, whatever was taught concerning the nature either of the human mind or of the Deity, made a part of the system of physics. Those beings, in whatever their essence might be supposed to consist, were parts of the great system of the universe, and parts, too, productive of the most important effects. Whatever human reason could either conclude, or conjecture, concerning them made, as it were, two chapters, though no doubt two very important ones of the science which pretended to give an account of the origin and revolutions of the great system of the universe. But in the universities of Europe, where philosophy was taught only as subservient to theology, it was natural to dwell longer upon these two chapters than upon any other of the science. They were gradually more and more extended, and were divided into many inferior chapters, till at last the doctrine of spirits, of which so little can be known, came to take up as much room in the system of philosophy as the doctrine of bodies of which so much can be known. The doctrines concerning those two subjects were considered as making two distinct sciences. What are called Metaphysics or Pneumatics were set in opposition to Physics, and were cultivated not only as the more sublime, but, for the purposes of a particular profession, as the more useful science of the two. The proper subject of experiment and observation, a subject in which a careful attention is capable of making so many useful discoveries, was almost entirely neglected. The subject in which, after a few very simple and almost obvious truths, the most careful attention can discover nothing but obscurity and uncertainty, and can consequently produce nothing but subtleties and sophisms, was greatly cultivated.
