14. No answer can be successfully attempted in reply to the first of the questions which closed the last chapter until we have given precision to certain terms of incessant recurrence. I have often to remark on the peculiar misfortune of Psychology, that all its principal terms are employed by different writers, and are understood by different readers, in widely different senses: they denote and connote meanings of various significance. All physicists mean the same thing when they speak of weight, mass, momentum, electricity, heat, etc. All chemists mean the same thing when they speak of affinity, decomposition, oxygen, carbonic acid, etc. All physiologists mean the same thing when they speak of muscle, nerve, nutrition, secretion, etc. But scarcely any two psychologists mean precisely the same thing when they speak of sensation, feeling, thought, volition, consciousness, etc.; and the differences of denotation and connotation in their uses of such terms lead to endless misunderstanding. As Rousseau says: “Les dÉfinitions pourraient Être bonnes si l’on n’employait pas les mots pour les faire.” But since we must employ words as our signs, our utmost care should be given to clearly marking what it is the signs signify.

15. The question we have now before us, whether animal actions are interpretable on purely mechanical principles? can only be answered after a preliminary settlement of the terms. The first of these terms to be settled is that of mechanism, when applied to the vital organism. If the organism is a mechanism, its actions must of course be interpretable on mechanical principles. But this general truth requires a special interpretation, if on inquiry we find that the organism is a particular kind of mechanism, one which is not to be classed under the same head as inorganic machines. And this we do find. In Problem I. §22, will be found a statement of the radical difference between organic and inorganic mechanisms, due to the differences in their structures. But the differences there noted do not affect the operation of abstract mechanical principles, which are of course manifested wherever there is a dependent sequence of material changes; and which are the same abstract principles in the mechanism of the heavens, the mechanism of a paper-mill, or the mechanism of an animal body. In other words, the principles are abstract, and are abstracted from all concrete cases by letting drop what is special to each case, retaining only what is common to all. This procedure is indispensable to the ideal constructions of Science. But we cannot rightly interpret any concrete case by abstract principles alone; we must restore the special characters which the abstraction has eliminated. The most lucid explanation of the mechanism of the heavens will leave us quite in the dark respecting the action of a paper-mill, until we have studied the mill at work, ascertained its structure and mode of operation, and therein detected what is common both to its mechanism and to the mechanism of the heavens. Thus equipped, we approach the study of the animal mechanism, but find ourselves wholly in the dark until we have also ascertained its structure and mode of operation; then we may recognize in it the principles of dependent sequence which had been abstracted from the paper-mill and the heavens. To neglect this concrete study, and to argue from Machinery to Life in disregard of special conditions, is not more rational than to assume that the movement of a piston is prompted by volition.

16. The recognition of special differences is no denial of fundamental identities. We do not deny the presence of phenomena in organisms which belong to physical and chemical agencies, but we assert that organisms have other phenomena besides these, dependent on conditions not present in physical and chemical phenomena. The same material elements and forces may be recognized in a moving inorganic body, and a moving organic body; but in the latter there is a speciality of combination with a speciality of result. Just as the same words and laws of grammatical construction may be recognized in prose and poetry; yet poetry is not prose, but has special rules of its own, and special effects. In an organism, as in a machine, the adjustment of the parts is a condition of the mechanical action; the one enables us to explain the other. But the parts adjusted, and the consequences of the adjustment, are unlike in the two cases. This unlikeness is pervading and profound. One cardinal difference is that the combination of the parts is in the machine a fixed, in the organism a fluctuating adjustment; and this fluctuation is due to certain vital processes subjectively known as sensitive guidance. Hence machines have fixed and calculated mechanisms; whereas organisms are variable and to a great extent incalculable mechanisms.

17. I conceive, therefore, that a theory which reduces vital activities to purely physical processes is self-condemned. Not that we are to admit the agency of any extra-organic principle, such as the hypothesis of Vitalism assumes (Prob. I. §14); but only the agency of an intra-organic principle, or the abstract symbol of all the co-operant conditions—the special combination of forces which result in organization. This assures us that an organism is a peculiar kind of mechanism, the processes in which are peculiar to it; and among those processes there is one which results in what we call Sensibility. This Sensibility is a factor which raises the phenomena into another order. To overlook its presence is fatal to any explanation of the organic mechanism. Yet it is overlooked by those who tell us that when an impression on a nerve is conveyed to the brain, and is thence reflected on the limbs—as when the retina of a wolf is stimulated by the image of a sheep, and the spring of the wolf upon the sheep follows as a “purely mechanical consequence—the whole process has from first to last been physical.” Unless the term physical is here used to designate the objective sequence, as contemplated by an onlooker, who likens the process to the sequence observable in a machine, I should say that from first to last the process has been not physical, but vital, involving among its essential conditions the peculiarly vital factor named Sensibility. The process taking place in the wolf’s organism is one which involves conditions never found in purely physical processes. We may indeed analytically disregard these. We may view the process in its purely physical relations, or in its purely chemical relations, or in its purely mathematical (mechanical) relations. But this is the artifice of the analytical method. In reality the process is no one of these, for it is all of these; it is a process in a living organism, and depends on conditions only found in living organisms—nay, in this particular case the process depends on conditions only found in organisms like that of the wolf; for the image of the sheep will stimulate the brain of a goat, horse, or elephant without producing any such movement in the organism.

18. The importance of this point must excuse my reiteration of it. We must make clear to ourselves that the organism is in its objective aspect a physiological mechanism, in its subjective aspect a psychological mechanism: in both aspects it is to be radically demarcated from all inorganic mechanisms. In it the combination and co-ordination of movements involve conditions never present in machines; among these conditions, there are combinations and co-ordinations of Sensibility, which, although material processes on the objective side, are processes believed to be only present in organisms. We have the strongest reasons for concluding that every feeling, every change in Sensibility, has its correlative material process in the organism—is, in short, only the subjective aspect of the objective organic change. What in Physiology is called Co-ordination and has reference to movements, in Psychology may be called Logic, having reference to feelings. But be this latter point accepted or rejected, the one point which admits of no dispute is that an organism is radically distinguishable from every inorganic mechanism in that it acquires through the very exercise of its primary constitution, a new constitution with new powers. Its adjustment is a changing and developing mechanism. That is to say, a machine, however complex its structure, is constructed once for all, and this primary constitution is final, the adjustment of parts remaining unaltered; and although by exercise the machine may come to work more easily, with less friction, it never comes to work differently, to readjust its parts, and develop new capabilities. It has no historical factor manifest in its functions. It has no experience. It reacts at last as at first. How different the organism! This has not only variable adjustments due to internal fluctuations, it has experience which develops new parts, and new adjustments of old parts. Every organism has its primary constitution in the adjustment of parts peculiar to the species; it has also its secondary or modified constitution, in the adjustment which has been more or less altered by individual experiences; it has, thirdly, its temporary constitution in the variable adjustment due to the varying state of tension which results from varying stimulation.

19. A word on each. There is a structural disposition of the parts which is common to large groups of organisms, so that a corresponding similarity is observable in the reactions of these organisms. Thus all quadrupeds use their limbs for locomotion in very similar ways; birds use their wings for flight in similar ways. All vertebrates swallow their food, defend themselves, shrink when hurt, etc., in ways that are very similar. In so far as their organizations are alike, their actions and reactions are alike. In so far as their organizations differ, their actions and reactions differ. The goose and the vulture are alike in the main lines of structure; still more alike are duck and hen; yet, owing to certain unlike characters of structure, they manifest some marked differences in action and reaction: the goose will starve in the presence of food which the vulture gluttonously devours, and the vulture will refuse the vegetable food which the goose devours; the duck plunges into the water, the hen not only refuses to enter it, but is greatly agitated when she sees the ducklings she has hatched plunging into it. That peculiar instincts, habits, and feelings are rigorously determined by peculiarities in the organism, no one doubts, when animals are in question. If this is less obvious in the case of men, the reason is that there the influence of other factors somewhat masks the operation of the primary constitution—these factors are the modified and the temporary constitutions. Yet even in man it is true to say that his feelings and actions are the result of his organization, native and acquired.

20. No two men are organized in all respects alike. There are individual variations in structure, both native and acquired. These may be too slight to be appreciable by any other test than the difference of reaction under similar external stimuli; but the variations in the sensibility to music, color, temperature, sexual influence, moral influence, etc., betray corresponding differences in the organisms. Any one variation in structure, seemingly trivial, may be the origin of well-marked diversity in physical and moral characters. Compare the bull with the ox, or the predatory aggressive eagle with the cowardly vulture. Nor are the temporary modifications to be overlooked. Antoine Cros mentions the case of a patient, a young girl, suffering from congested liver and spleen, which of course altered the state of her blood, and thus for a time modified her constitution. Her moral character was greatly altered by it. She ceased to feel any affection for father or mother; would play with her doll, but could not be brought to show any delight in it; could not be drawn out of her apathetic sadness. Things which previously had made her shriek with laughter, now left her uninterested. Her temper changed, became capricious and violent.207 Congestion of the lungs, if unaccompanied by congestion of the liver, never produces such effects, because not thus altering the blood. The effects of liver congestion are familiar. Cros cites the case of a magistrate whose liver was enlarged, and whose skin showed a markedly bilious aspect, and in whom all affection seemed to be dead: he did not exhibit any perversion or violence, only want of emotive reaction. If he went to the theatre he could not feel the slightest pleasure in it. The thoughts of his home, his absent wife and children, were, he declared, as unaffecting to him as a problem in Euclid.

21. Owing to the recognized dependence of peculiar instincts and modes of reaction on peculiarities of structure, comparative anatomists are quite confident, when they find a portion of a skull with two occipital condyles, that the animal to which this skull belonged had red blood-corpuscles without nuclei, and (if a female) suckled its young. If in that fragment of skull there remain a single tooth, it will prove that the animal was carnivorous or herbivorous, and had, or had not, retractile claws. From such data a general conclusion may be formed as to the instincts and habits of the animal. The data disclose much of the primary constitution, that is to say, the mechanism which the animal brought with it into the world, ready prepared to react in definite ways on being stimulated. The connate mechanism has correlative tendencies of reaction. Some of these tendencies are inevitably called into play by external conditions, and they continue unaltered amid great varieties of circumstances, provided none of these variations directly deprive them of their appropriate stimulation. Such tendencies of the connate mechanism are styled automatic (an unfortunate metaphor, which has led to the theory of Automatism), and include, besides the visceral reactions, the more complex reactions of winking, breathing, swallowing, coughing, flying, walking, etc. It is true that we learn to walk, and learn to wink, whereas the other actions require no tentative efforts directed by experience; but the mechanism of all these actions is already laid down in the primary constitution, and is inevitably called into play.

22. The instincts also belong to the connate mechanism, and in the course of the normal experience of the animal inevitably come into play; but, unlike the automatic tendencies of breathing, swallowing, and coughing, they are capable of modification, or even suppression, by alterations in the course of individual experience. The connate mechanism of the cat determines its dread of water, and its enmity to the dog and mouse; yet a cat will by the modifications of certain experiences become as ready as an otter to take to the water, and become so fond of a dog that she will allow him to tend upon her kittens; and so indifferent to the mouse that she will let it run over her body. All this implies a new adjustment in the nervous centres, with new modes of reaction on sensory impressions: the inherited mechanism has been modified. I need not dwell on the profound modifications which the human inherited mechanism undergoes in the course of experience—how social influences and moral and religious teachings redirect, or even suppress, many primary tendencies; so that “moral habits” become organized, and replace the original tendencies of the organism. These, when organized, become the inevitable modes of reaction, and are sometimes called secondarily automatic. It is important to recognize this organization of experiences, this acquisition of a secondary or modified constitution, if we would explain psychological processes by physiological processes. Thus the processes of Logic are automatic, they belong to the connate primary mechanism, and their action is inevitable, invariable. The elements of a judgment, like the elements of a perception, may vary, and we therefore say that one judgment is false, and one perception incomplete; but the judging process is always the same, and the perceiving process is always the same. We may breathe pure air or impure air, but the breathing process is in each case the same; and judgment is as automatic as breathing, not to be altered, not to be suppressed. Again, the moral terror at wickedness of any recognized kind is as automatic as the instinctive terror at danger. The one has its roots in the primary disposition called love of approbation and its correlative dread of disapprobation: the social instinct. The other has its root in the primary disposition called “instinct of self-preservation,” which is really the reflex shrinking from pain: the physiological instinct. 23. Besides the connate and acquired mechanism, we have now to consider the temporary and fluctuating adjustments which represent the statical condition of the organism at each moment. The automatism of the primary constitution is such that previous experience and conscious effort are not needed; nor will any experience or any effort alter the mode of reaction. If a strong light falls on the eye, the iris contracts; if the eyeball is dry, the eyelid drops; if sound-waves beat upon the tympanum, the stapedius muscle contracts; if the lining of the throat be tickled, the muscles involved in coughing or in vomiting contract. No experience is necessary for these actions, some of which are so complicated that if we had to learn them, as we learn far simpler actions, the organism would perish before the power was attained. Yet all of these presuppose a certain normal state of the mechanism, any considerable variation in which will modify or suppress them.

24. Secondarily automatic actions are those which have been acquired through experiences that have modified the organism, and produced a new adjustment of parts. We learn to shield the eyes against a strong glare of light by raising the hand; by winking we learn to shield the eye against an approaching body; we also learn to turn the head in the direction of a sound, and to thrust away with our hands the object that is irritating our skin. Experience has been necessary for all these actions, and has finally organized the tendencies to perform them, so that the reaction is invariable, inevitable, unless controlled by the will. If you tickle my throat, I may, or may not, push aside your hand; but if the inside of my throat be tickled, I must cough. Here we see the difference between the automatic and secondarily automatic actions. The second being due to individual experience, are more or less controllable; and whether they are or are not controlled depends on the condition of the nerve-centres at the moment. You may tickle my throat, or irritate my skin, without causing any movement of my hands to thwart you, either because my nerve-centres are preoccupied by other stimulations, and I am not conscious of the irritation, or because I do not choose to thwart you.

25. It should be added that some secondarily automatic actions have become so firmly organized that we can only with great difficulty interfere with them. Others never enter into consciousness, and are therefore often supposed to be purely mechanical. The movement of the eye towards the brightest light, and the convergence of the axes of both eyes, are reflexes which, although involuntary and unconscious, are the products of education. They do not belong to the connate constitution, although they are so inevitably acquired by experience that they belong to every normal child. At first the infant stares with a blank gaze, and its eyes, though moving under the stimulus of light, move incoherently; the axes never converge except by accident. Very early, however, the infant’s eyes are observed to follow the movements of a bright light; and at last they acquire so certain and rapid a power of adjustment that the eyes shift from spot to spot, always “fixing” the object by bringing the most sensitive part of the retina to bear on it. The incoherent movements have become precisely regulated movements. It is the same with speech. The vocal organs are exercised in an incoherent babble. By degrees these movements become regulated so as to respond definitely to definite stimuli, and words are formed, then sentences, till finally fluent speech becomes in a great degree automatic. The vocal muscles respond to an auditory stimulus, and the child repeats the word it has heard, just as the eye-muscles respond to a retinal stimulus. That we acquire the power of converging the axes, and accommodating the lens to near objects, is not only proved by observation of infants, but also by cases of disease. After the reflex mechanism has been long established, so that it acts with inevitable precision, a slight paralysis of one of the muscles has the effect of making all objects appear in a different position; the patient trying to touch an object, then always moves his hand on one side of it. Von Graefe relates the case of a stonebreaker who always struck his hand with the hammer when he tried to strike the stone. Yet this very man learned to accommodate his movements to the new impressions; so that if his paralysis had been cured, his modified mechanism would have been ill adapted to the new conditions, and he would once more have struck his hand instead of the stone.

26. This digression on the native and acquired dispositions of the organism, while it has brought into strong light all that can be cited in favor of regarding animal bodies as mechanisms, and their actions as the direct consequences of mechanical adjustments, has also made conspicuous the radical difference between an organism and a machine. We cannot too emphatically insist on this radical difference. Between the group of conditions involved in the structure and action of a machine, and the group of conditions involved in the structure and action of an organism, there are contrasts as broad as any that can be named. To overlook these in taking account solely of the conditions common to both groups is a serious error. On such grounds we might insist that a tiger is a violet, because both are organisms.

The biologist will admit that an organism is a mechanism, and (in so far as its bodily structure is concerned) a material mechanism. All the actions of this structure are therefore mechanical, in the two senses of the term: first, as being the actions of material adjustments; secondly, as being movements, and thereby included under the general laws of motion represented in Mechanics; the abstract laws of movement for an organic body are not different from the abstract laws of movement for an inorganic body. So far we have been considering the abstract relations only. No sooner do we consider the phenomena as concrete wholes, than we find great diversity in the modes of production of the movements in organisms and machines. Now it is precisely the modes of production which have interest for us. We never understand a phenomenon so as to gain any practical control over it, or any theoretical illumination from it, unless we have mastered some of its conditions; our knowledge of these conditions is the measure of our power.