Notwithstanding the great mobility with which man's organization renders him susceptible, although exterior as well as interior causes are continually acting upon him, he does not always feel in a distinct, in a decided manner, the impulse given to his senses: indeed, he does not feel it, until it has produced some change, or given some shock to his brain. Thus, although completely environed by air, he does not feel its action, until it is so modified, as to strike with a sufficient degree of force on his organs; to penetrate his skin, through which his brain is warned of its presence. Thus, during a profound and tranquil sleep, undisturbed by any dream, man ceases to feel. In short, notwithstanding the continued motion that agitates his frame, man does not appear to feel, when this motion acts in a convenient order; he does not perceive a state of health, but he discovers a state of grief or sickness; because, in the first, his brain does not receive too lively an impulse, whilst in the others, his nerves are contracted, shocked, and agitated, with violent, with disorderly motion: these communicating with his brain, give notice that some cause acts strongly upon them--impels them in a manner that bears no analogy with their natural habit: this constitutes, in him, that peculiar mode of existing which he calls _grief_.
On the other hand, it sometimes happens that exterior objects produce very considerable changes on his body, without his perceiving them at the moment. Often, in the heat of battle, the soldier perceives not that he is dangerously wounded, because, at the time, the rapidity, the multiplicity of impetuous motion that assails his brain, does not permit him to distinguish the particular change a part of his body has undergone by the wound. In short, when a great number of causes are simultaneously acting on him with too much vivacity, he sinks under their accumulated pressure,--he swoons--he loses his senses--he is deprived of feeling.
In general, feeling only obtains, when the brain can distinguish distinctly, the impressions made on the organs with which it has communication; it is the distinct shock, the decided modification man undergoes, that constitutes _conscience_. Doctor Clarke, says to this effect: "Conscience is the act of reflecting, by means of which I know that I think, and that my thoughts, or my actions belong to me, and not to another." From this it will appear, that _feeling_ is a mode of being, a marked change, produced on our brain, occasioned by the impulse communicated to our organs, whether by interior or exterior agents, by which it is modified either in a durable or transient manner: it is not always requisite that man's organs should be moved by an exterior object, to enable him to feel that he should be conscious of the changes effected in him: he can feel them within himself by means of an interior impulse; his brain is then modified, or rather he renews within himself the anterior modifications. We are not to be astonished that the brain should be necessarily warned of the shocks, of the impediments, of the changes that may happen to so complicated a machine as the human body, in which, notwithstanding all the parts are contiguous to the brain, and concentrate themselves in this brain, and are by their essence in a continual state of action and re-action.