66. There is no real and essential distinction between voluntary and involuntary actions. They all spring from Sensibility. They are all determined by feeling. It is convenient, for common purposes, to designate some actions as voluntary; but this is merely a convenience; no psychological nor physiological insight is gained by it; an analysis of the process discloses no element in a voluntary action which is not to be found in an involuntary action--except in the origin or degree of stimulation. In ordinary language it is convenient to mark a distinction between my raising my arm because I will to raise it for some definite purpose, and my raising it because a bee has stung me; it is convenient to say, “I _will_ to write this letter,” and “this letter is written against my will--I have no will in the matter.” But Science is more exacting when it aims at being exact; and the philosopher, analyzing these complex actions, will find that in each case certain muscular groups have been set in action by different sensational or ideational stimuli. The action itself is that of a neuro-muscular mechanism, which mechanism works in the same way, whatever be the source of the original impulse. The stimulation may be incited directly from the periphery, or indirectly from a remote centre; and the action may be arrested by a peripheral or central stimulation: the reflex which ordinarily follows the excitation of a sensory nerve will be modified, or arrested, if some other nerve be at the same time stimulated. (See Law of Arrest, _Prob._ II. § 190.) 67. All actions are reflex, all are the operations of a mechanism, all are sentient, because the mechanism has Sensibility as its vital property. In thus preserving the integrity of the order of vital phenomena, and keeping them classified apart from physical and chemical phenomena, we by no means set aside the useful distinctions expressed in the terms voluntary and involuntary; any more than we set aside the distinction of vertebrate and invertebrate when both are classed under Animal, and separated from Plant, or Planet.