For my own part, I am persuaded that no Theory of the mind can be satisfactory, which is not founded on the history of the Body. I know of no legitimate passport to Metaphysics but Physiology. Hence I cannot estimate highly the writings of the Scotch Metaphysicians. There is one other feature also common to this School, which satisfies me of their incompetence to this subject; their slight notice, and ambiguous approbation of a man so superior as Dr. Hartley, and their utter ignorance or neglect of the theory he has advanced. On every subject relating to the phenomena of mind, Dr. Hartley’s book must be adopted as the ground work of the reasoning, or his principles must be previously and distinctly confuted.[76] [76] Dr. Reid in his last work has given a critique on Dr. Hartley’s theory without understanding it, or even touching on the important points. That theory in substance is this: an external object (a peach for instance) makes an impression at once, on our organs of feeling, of sight, and of taste. The impression thus made on the extreme end of the appropriate nerve, is propagated by some species of motion along the course of nerve up to the brain, and there, and there only, perceived; for if the nerve be cut, or tied, or palsied, in any part of its course, the impression is not perceived. Motions in the brain thus produced, and perceived, are _sensations_: similar motions arising, or produced without the impression of an external object, are _ideas_. These impressions being in the instance given, simultaneous or nearly so, are associated, so that the sensation produced by the sight of a peach, will give rise to motions in the brain similar to those produced at first by the taste and the touch of it: i. e. it will suggest the _ideas_ of taste and touch, and excite the inclination to reach and to eat the object of them. Hence sensations, ideas, and muscular motions are associated together and mutually suggest and give rise to each other. What species of motion it is, with which the nervous system is affected in this process, or whether Sir Isaac Newton’s Æther, or its modern substitute the electric fluid, has any thing to do with it or not, is no essential part of the theory, and may be adopted or rejected without prejudice to the main system. Some kind of motion there manifestly is; I think it _demonstrable_ that it is vibratory; but of whatever kind it be, its existence in the brain is unquestionable; and the association and catenation of individual motions in the brain according to certain laws, is equally so. This is matter of fact, and it was Dr. Reid’s business if he could, to shew that neither the motions, the perceptions, or the associations took place in that organ. The general law is expressed by Hartley Prop. 20. Cor. 7.