The doctrine of necessity being admitted, it follows that the theory of the human mind is properly, like the theory of every other series of events with which we are acquainted, a system of mechanism; understanding by mechanism nothing more than a regular succession of phenomena, without any uncertainty of event, so that every consequent requires a specific antecedent, and could be no otherwise in any respect than as the antecedent determined it to be.
But there are two sorts of mechanism capable of being applied to the solution of this case, one which has for its medium only matter and motion, the other which has for its medium thought. Which of these is to be regarded as most probable?
According to the first, we may conceive the human body to be so constituted as to be susceptible of vibrations, in the same manner as the strings of a musical instrument. These vibrations, having begun upon the surface of the body, are conveyed to the brain; and, in a manner that is equally the result of construction, produce a second set of vibrations beginning in the brain, and conveyed to the different organs or members of the body.
Thus it may be supposed that a piece of iron considerably heated is applied to the body of an infant, and that the report of this irritation and separation of parts being conveyed to the brain vents itself again in a shrill and piercing cry. It is in this manner that certain convulsive and spasmodic affections appear to take place in the body. The case, as here described, is similar to that of the bag of a pair of bagpipes, which, being pressed in a certain manner, utters a groan, without anything more being necessary to account for this phenomenon than the known laws of matter and motion.
Let us add to these vibrations a system of associations to be carried on by traces to be made upon the medullary substance of the brain, by means of which past and present impressions are connected according to certain laws, as the traces happen to approach or run into each other; and we have then a complete scheme of a certain sort, of the phenomena of human action.
It is to be observed that, according to this system, mind, or perception, is altogether unnecessary to explain the appearances. It might for other reasons be desirable or wise, in the author of the universe for example, to introduce a thinking substance, or a power of perception, as a spectator of the process. But this percipient power is altogether neutral, having apparently no concern, either as a medium or otherwise, in the events to be produced.
The second system, which represents thought as the medium of operation, is not less a system of mechanism according to the doctrine of necessity, but it is a mechanism of a totally different kind.
There are various reasons calculated to persuade us that this last hypothesis is the most profitable. No inconsiderable argument may be derived from the singular and important nature of that property of human beings which we term thought; which it is surely somewhat violent to strike out of our system, as a mere superfluity.
A second reason still more decisive than the former arises from the constancy with which thought, in innumerable instances, accompanies the functions of this mechanism. Now this constancy of conjunction has been shown to be the only ground we have, in any imaginable subject, for proceeding from antecedent to consequent, and expecting, when we see one given event, that another event of a given sort will succeed it. We cannot therefore reject the principle which supposes thought to be a real medium in the mechanism of man, but upon grounds that would vitiate our reasonings in every topic of human enquiry.
It may be objected 'that, though this regularity of event is the only rational principle of inference, yet thought may be found not to possess the character of a medium, motion being in all instances the antecedent, and thought never anything more than a consequent'. But this is contrary to everything we know of the system of the universe, in which each event appears to be alternately both the one and the other, nothing terminating in itself, but everything leading on to an endless chain of consequences.
It would be equally vain to object 'that we are unable to conceive how thought can have any tendency to promotion in the animal system'; since it has just appeared that this ignorance is by no means peculiar to the subject before us. We are universally unable to perceive a foundation of necessary connection.
It being then sufficiently clear that there are cogent reasons to persuade us that thought is the medium through which the motions of the animal system are generally carried on, let us proceed to consider what is the nature of those thoughts by which the limbs and organs of our body are set in motion. It will then probably be found that the difficulties which have clogged the intellectual hypothesis are principally founded in erroneous notions derived from the system of liberty; as if there were any essential difference between those thoughts which are the medium of generating motion, and thoughts in general.
First, thought may be the source of animal motion, without partaking, in any degree, of volition, or design. It is certain that there is a great variety of motions in the animal system which are, in every view of the subject, involuntary. Such, for example, are the cries of an infant, when it is first impressed with the sensation of pain. In the first motions of the animal system, nothing of any sort could possibly be foreseen, and therefore nothing of any sort could be intended. Yet these motions have sensation or thought for their constant concomitant; and therefore all the arguments which have been already alleged remain in full force, to prove that thought is the medium of their production.