In attempting to understand the elements out of which mentalphenomena are compounded, it is of the greatest importance toremember that from the protozoa to man there is nowhere a verywide gap either in structure or in behaviour. From this fact itis a highly probable inference that there is also nowhere a verywide mental gap. It is, of course, POSSIBLE that there may be, atcertain stages in evolution, elements which are entirely new fromthe standpoint of analysis, though in their nascent form theyhave little influence on behaviour and no very markedcorrelatives in structure. But the hypothesis of continuity inmental development is clearly preferable if no psychologicalfacts make it impossible. We shall find, if I am not mistaken,that there are no facts which refute the hypothesis of mentalcontinuity, and that, on the other hand, this hypothesis affordsa useful test of suggested theories as to the nature of mind.

The hypothesis of mental continuity throughout organic evolutionmay be used in two different ways. On the one hand, it may beheld that we have more knowledge of our own minds than those ofanimals, and that we should use this knowledge to infer theexistence of something similar to our own mental processes inanimals and even in plants. On the other hand, it may be heldthat animals and plants present simpler phenomena, more easilyanalysed than those of human minds; on this ground it may beurged that explanations which are adequate in the case of animalsought not to be lightly rejected in the case of man. Thepractical effects of these two views are diametrically opposite:the first leads us to level up animal intelligence with what webelieve ourselves to know about our own intelligence, while thesecond leads us to attempt a levelling down of our ownintelligence to something not too remote from what we can observein animals. It is therefore important to consider the relativejustification of the two ways of applying the principle ofcontinuity.

It is clear that the question turns upon another, namely, whichcan we know best, the psychology of animals or that of humanbeings? If we can know most about animals, we shall use thisknowledge as a basis for inference about human beings; if we canknow most about human beings, we shall adopt the oppositeprocedure. And the question whether we can know most about thepsychology of human beings or about that of animals turns uponyet another, namely: Is introspection or external observation thesurer method in psychology? This is a question which I propose todiscuss at length in Lecture VI; I shall therefore content myselfnow with a statement of the conclusions to be arrived at.

We know a great many things concerning ourselves which we cannotknow nearly so directly concerning animals or even other people.We know when we have a toothache, what we are thinking of, whatdreams we have when we are asleep, and a host of otheroccurrences which we only know about others when they tell us ofthem, or otherwise make them inferable by their behaviour. Thus,so far as knowledge of detached facts is concerned, the advantageis on the side of self-knowledge as against external observation.

But when we come to the analysis and scientific understanding ofthe facts, the advantages on the side of self-knowledge becomefar less clear. We know, for example, that we have desires andbeliefs, but we do not know what constitutes a desire or abelief. The phenomena are so familiar that it is difficult torealize how little we really know about them. We see in animals,and to a lesser extent in plants, behaviour more or less similarto that which, in us, is prompted by desires and beliefs, and wefind that, as we descend in the scale of evolution, behaviourbecomes simpler, more easily reducible to rule, morescientifically analysable and predictable. And just because weare not misled by familiarity we find it easier to be cautious ininterpreting behaviour when we are dealing with phenomena remotefrom those of our own minds: Moreover, introspection, aspsychoanalysis has demonstrated, is extraordinarily fallible evenin cases where we feel a high degree of certainty. The net resultseems to be that, though self-knowledge has a definite andimportant contribution to make to psychology, it is exceedinglymisleading unless it is constantly checked and controlled by thetest of external observation, and by the theories which suchobservation suggests when applied to animal behaviour. On thewhole, therefore, there is probably more to be learnt about humanpsychology from animals than about animal psychology from humanbeings; but this conclusion is one of degree, and must not bepressed beyond a point.

It is only bodily phenomena that can be directly observed inanimals, or even, strictly speaking, in other human beings. Wecan observe such things as their movements, their physiologicalprocesses, and the sounds they emit. Such things as desires andbeliefs, which seem obvious to introspection, are not visibledirectly to external observation. Accordingly, if we begin ourstudy of psychology by external observation, we must not begin byassuming such things as desires and beliefs, but only such thingsas external observation can reveal, which will be characteristicsof the movements and physiological processes of animals. Someanimals, for example, always run away from light and hidethemselves in dark places. If you pick up a mossy stone which islightly embedded in the earth, you will see a number of smallanimals scuttling away from the unwonted daylight and seekingagain the darkness of which you have deprived them. Such animalsare sensitive to light, in the sense that their movements areaffected by it; but it would be rash to infer that they havesensations in any way analogous to our sensations of sight. Suchinferences, which go beyond the observable facts, are to beavoided with the utmost care.

It is customary to divide human movements into three classes,voluntary, reflex and mechanical. We may illustrate thedistinction by a quotation from William James ("Psychology," i,12):

"If I hear the conductor calling 'all aboard' as I enter thedepot, my heart first stops, then palpitates, and my legs respondto the air-waves falling on my tympanum by quickening theirmovements. If I stumble as I run, the sensation of fallingprovokes a movement of the hands towards the direction of thefall, the effect of which is to shield the body from too sudden ashock. If a cinder enter my eye, its lids close forcibly and acopious flow of tears tends to wash it out.

"These three responses to a sensational stimulus differ, however,in many respects. The closure of the eye and the lachrymation arequite involuntary, and so is the disturbance of the heart. Suchinvoluntary responses we know as 'reflex' acts. The motion of thearms to break the shock of falling may also be called reflex,since it occurs too quickly to be deliberately intended. Whetherit be instinctive or whether it result from the pedestrianeducation of childhood may be doubtful; it is, at any rate, lessautomatic than the previous acts, for a man might by consciouseffort learn to perform it more skilfully, or even to suppress italtogether. Actions of this kind, with which instinct andvolition enter upon equal terms, have been called 'semi-reflex.'The act of running towards the train, on the other hand, has noinstinctive element about it. It is purely the result ofeducation, and is preceded by a consciousness of the purpose tobe attained and a distinct mandate of the will. It is a'voluntary act.' Thus the animal's reflex and voluntaryperformances shade into each other gradually, being connected byacts which may often occur automatically, but may also bemodified by conscious intelligence.

"An outside observer, unable to perceive the accompanyingconsciousness, might be wholly at a loss to discriminate betweenthe automatic acts and those which volition escorted. But if thecriterion of mind's existence be the choice of the proper meansfor the attainment of a supposed end, all the acts alike seem tobe inspired by intelligence, for APPROPRIATENESS characterizesthem all alike."

There is one movement, among those that James mentions at first,which is not subsequently classified, namely, the stumbling. Thisis the kind of movement which may be called "mechanical"; it isevidently of a different kind from either reflex or voluntarymovements, and more akin to the movements of dead matter. We maydefine a movement of an animal's body as "mechanical" when itproceeds as if only dead matter were involved. For example, ifyou fall over a cliff, you move under the influence ofgravitation, and your centre of gravity describes just as correcta parabola as if you were already dead. Mechanical movements havenot the characteristic of appropriateness, unless by accident, aswhen a drunken man falls into a waterbutt and is sobered. Butreflex and voluntary movements are not ALWAYS appropriate, unlessin some very recondite sense. A moth flying into a lamp is notacting sensibly; no more is a man who is in such a hurry to gethis ticket that he cannot remember the name of his destination.Appropriateness is a complicated and merely approximate idea, andfor the present we shall do well to dismiss it from our thoughts.

As James states, there is no difference, from the point of viewof the outside observer, between voluntary and reflex movements.The physiologist can discover that both depend upon the nervoussystem, and he may find that the movements which we callvoluntary depend upon higher centres in the brain than those thatare reflex. But he cannot discover anything as to the presence orabsence of "will" or "consciousness," for these things can onlybe seen from within, if at all. For the present, we wish to placeourselves resolutely in the position of outside observers; wewill therefore ignore the distinction between voluntary andreflex movements. We will call the two together "vital"movements. We may then distinguish "vital" from mechanicalmovements by the fact that vital movements depend for theircausation upon the special properties of the nervous system,while mechanical movements depend only upon the properties whichanimal bodies share with matter in general.

There is need for some care if the distinction between mechanicaland vital movements is to be made precise. It is quite likelythat, if we knew more about animal bodies, we could deduce alltheir movements from the laws of chemistry and physics. It isalready fairly easy to see how chemistry reduces to physics, i.e.how the differences between different chemical elements can beaccounted for by differences of physical structure, theconstituents of the structure being electrons which are exactlyalike in all kinds of matter. We only know in part how to reducephysiology to chemistry, but we know enough to make it likelythat the reduction is possible. If we suppose it effected, whatwould become of the difference between vital and mechanicalmovements?

Some analogies will make the difference clear. A shock to a massof dynamite produces quite different effects from an equal shockto a mass of steel: in the one case there is a vast explosion,while in the other case there is hardly any noticeabledisturbance. Similarly, you may sometimes find on a mountain-sidea large rock poised so delicately that a touch will set itcrashing down into the valley, while the rocks all round are sofirm that only a considerable force can dislodge them What isanalogous in these two cases is the existence of a great store ofenergy in unstable equilibrium ready to burst into violent motionby the addition of a very slight disturbance. Similarly, itrequires only a very slight expenditure of energy to send apost-card with the words "All is discovered; fly!" but the effectin generating kinetic energy is said to be amazing. A human body,like a mass of dynamite, contains a store of energy in unstableequilibrium, ready to be directed in this direction or that by adisturbance which is physically very small, such as a spokenword. In all such cases the reduction of behaviour to physicallaws can only be effected by entering into great minuteness; solong as we confine ourselves to the observation of comparativelylarge masses, the way in which the equilibrium will be upsetcannot be determined. Physicists distinguish between macroscopicand microscopic equations: the former determine the visiblemovements of bodies of ordinary size, the latter the minuteoccurrences in the smallest parts. It is only the microscopicequations that are supposed to be the same for all sorts ofmatter. The macroscopic equations result from a process ofaveraging out, and may be different in different cases. So, inour instance, the laws of macroscopic phenomena are different formechanical and vital movements, though the laws of microscopicphenomena may be the same.

We may say, speaking somewhat roughly, that a stimulus applied tothe nervous system, like a spark to dynamite, is able to takeadvantage of the stored energy in unstable equilibrium, and thusto produce movements out of proportion to the proximate cause.Movements produced in this way are vital movements, whilemechanical movements are those in which the stored energy of aliving body is not involved. Similarly dynamite may be exploded,thereby displaying its characteristic properties, or may (withdue precautions) be carted about like any other mineral. Theexplosion is analogous to vital movements, the carting about tomechanical movements.

Mechanical movements are of no interest to the psychologist, andit has only been necessary to define them in order to be able toexclude them. When a psychologist studies behaviour, it is onlyvital movements that concern him. We shall, therefore, proceed toignore mechanical movements, and study only the properties of theremainder.

The next point is to distinguish between movements that areinstinctive and movements that are acquired by experience. Thisdistinction also is to some extent one of degree. Professor LloydMorgan gives the following definition of "instinctive behaviour":

"That which is, on its first occurrence, independent of priorexperience; which tends to the well-being of the individual andthe preservation of the race; which is similarly performed by allmembers of the same more or less restricted group of animals; andwhich may be subject to subsequent modification under theguidance of experience."^[1]

This definition is framed for the purposes of biology, and is insome respects unsuited to the needs of psychology. Though perhapsunavoidable, allusion to "the same more or less restricted groupof animals" makes it impossible to judge what is instinctive inthe behaviour of an isolated individual. Moreover, "thewell-being of the individual and the preservation of the race" isonly a usual characteristic, not a universal one, of the sort ofmovements that, from our point of view, are to be calledinstinctive; instances of harmful instincts will be givenshortly. The essential point of the definition, from our point ofview, is that an instinctive movement is in dependent of priorexperience.

We may say that an "instinctive" movement is a vital movementperformed by an animal the first time that it finds itself in anovel situation; or, more correctly, one which it would performif the situation were novel.^[2] The instincts of an animal aredifferent at different periods of its growth, and this fact maycause changes of behaviour which are not due to learning. Thematuring and seasonal fluctuation of the sex-instinct affords agood illustration. When the sex-instinct first matures, thebehaviour of an animal in the presence of a mate is differentfrom its previous behaviour in similar circumstances, but is notlearnt, since it is just the same if the animal has neverpreviously been in the presence of a mate.

On the other hand, a movement is "learnt," or embodies a "habit,"if it is due to previous experience of similar situations, and isnot what it would be if the animal had had no such experience.

There are various complications which blur the sharpness of thisdistinction in practice. To begin with, many instincts maturegradually, and while they are immature an animal may act in afumbling manner which is very difficult to distinguish fromlearning. James ("Psychology," ii, 407) maintains that childrenwalk by instinct, and that the awkwardness of their firstattempts is only due to the fact that the instinct has not yetripened. He hopes that "some scientific widower, left alone withhis offspring at the critical moment, may ere long test thissuggestion on the living subject." However this may be, he quotesevidence to show that "birds do not LEARN to fly," but fly byinstinct when they reach the appropriate age (ib., p. 406). Inthe second place, instinct often gives only a rough outline ofthe sort of thing to do, in which case learning is necessary inorder to acquire certainty and precision in action. In the thirdplace, even in the clearest cases of acquired habit, such asspeaking, some instinct is required to set in motion the processof learning. In the case of speaking, the chief instinct involvedis commonly supposed to be that of imitation, but this may bequestioned. (See Thorndike's "Animal Intelligence," p. 253 ff.)

In spite of these qualifications, the broad distinction betweeninstinct and habit is undeniable. To take extreme cases, everyanimal at birth can take food by instinct, before it has hadopportunity to learn; on the other hand, no one can ride abicycle by instinct, though, after learning, the necessarymovements become just as automatic as if they were instinctive.

The process of learning, which consists in the acquisition ofhabits, has been much studied in various animals.^[3] For example:you put a hungry animal, say a cat, in a cage which has a doorthat can be opened by lifting a latch; outside the cage you putfood. The cat at first dashes all round the cage, making franticefforts to force a way out. At last, by accident, the latch islifted. and the cat pounces on the food. Next day you repeat theexperiment, and you find that the cat gets out much more quicklythan the first time, although it still makes some randommovements. The third day it gets out still more quickly, andbefore long it goes straight to the latch and lifts it at once.Or you make a model of the Hampton Court maze, and put a rat inthe middle, assaulted by the smell of food on the outside. Therat starts running down the passages, and is constantly stoppedby blind alleys, but at last, by persistent attempts, it getsout. You repeat this experiment day after day; you measure thetime taken by the rat in reaching the food; you find that thetime rapidly diminishes, and that after a while the rat ceases tomake any wrong turnings. It is by essentially similar processesthat we learn speaking, writing, mathematics, or the governmentof an empire.

Professor Watson ("Behavior," pp. 262-3) has an ingenious theoryas to the way in which habit arises out of random movements. Ithink there is a reason why his theory cannot be regarded asalone sufficient, but it seems not unlikely that it is partlycorrect. Suppose, for the sake of simplicity, that there are justten random movements which may be made by the animal--say, tenpaths down which it may go--and that only one of these leads tofood, or whatever else represents success in the case inquestion. Then the successful movement always occurs during theanimal's attempts, whereas each of the others, on the average,occurs in only half the attempts. Thus the tendency to repeat aprevious performance (which is easily explicable without theintervention of "consciousness") leads to a greater emphasis onthe successful movement than on any other, and in time causes italone to be performed. The objection to this view, if taken asthe sole explanation, is that on improvement ought to set in tillafter the SECOND trial, whereas experiment shows that already atthe second attempt the animal does better than the first time.Something further is, therefore, required to account for thegenesis of habit from random movements; but I see no reason tosuppose that what is further required involves "consciousness."

Mr. Thorndike (op. cit., p. 244) formulates two "provisional lawsof acquired behaviour or learning," as follows:

"The Law of Effect is that: Of several responses made to the samesituation, those which are accompanied or closely followed bysatisfaction to the animal will, other things being equal, bemore firmly connected with the situation, so that, when itrecurs, they will be more likely to recur; those which areaccompanied or closely followed by discomfort to the animal will,other things being equal, have their connections with thatsituation weakened, so that, when it recurs, they will be lesslikely to occur. The greater the satisfaction or discomfort, thegreater the strengthening or weakening of the bond.

"The Law of Exercise is that: Any response to a situation will,other things being equal, be more strongly connected with thesituation in proportion to the number of times it has beenconnected with that situation and to the average vigour andduration of the connections."

With the explanation to be presently given of the meaning of"satisfaction" and "discomfort," there seems every reason toaccept these two laws.

What is true of animals, as regards instinct and habit, isequally true of men. But the higher we rise in the evolutionaryscale, broadly speaking, the greater becomes the power oflearning, and the fewer are the occasions when pure instinct isexhibited unmodified in adult life. This applies with great forceto man, so much so that some have thought instinct less importantin the life of man than in that of animals. This, however, wouldbe a mistake. Learning is only possible when instinct suppliesthe driving-force. The animals in cages, which gradually learn toget out, perform random movements at first, which are purelyinstinctive. But for these random movements, they would neveracquire the experience which afterwards enables them to producethe right movement. (This is partly questioned byHobhouse^[4]--wrongly, I think.) Similarly, children learning to talk make allsorts of sounds, until one day the right sound comes by accident.It is clear that the original making of random sounds, withoutwhich speech would never be learnt, is instinctive. I think wemay say the same of all the habits and aptitudes that we acquirein all of them there has been present throughout some instinctiveactivity, prompting at first rather inefficient movements, butsupplying the driving force while more and more effective methodsare being acquired. A cat which is hungry smells fish, and goesto the larder. This is a thoroughly efficient method when thereis fish in the larder, and it is often successfully practised bychildren. But in later life it is found that merely going to thelarder does not cause fish to be there; after a series of randommovements it is found that this result is to be caused by goingto the City in the morning and coming back in the evening. No onewould have guessed a priori that this movement of a middle-agedman's body would cause fish to come out of the sea into hislarder, but experience shows that it does, and the middle-agedman therefore continues to go to the City, just as the cat in thecage continues to lift the latch when it has once found it. Ofcourse, in actual fact, human learning is rendered easier, thoughpsychologically more complex, through language; but at bottomlanguage does not alter the essential character of learning, orof the part played by instinct in promoting learning. Language,however, is a subject upon which I do not wish to speak until alater lecture.

The popular conception of instinct errs by imagining it to beinfallible and preternaturally wise, as well as incapable ofmodification. This is a complete delusion. Instinct, as a rule,is very rough and ready, able to achieve its result underordinary circumstances, but easily misled by anything unusual.Chicks follow their mother by instinct, but when they are quiteyoung they will follow with equal readiness any moving objectremotely resembling their mother, or even a human being (James,"Psychology," ii, 396). Bergson, quoting Fabre, has made playwith the supposed extraordinary accuracy of the solitary waspAmmophila, which lays its eggs in a caterpillar. On this subjectI will quote from Drever's "Instinct in Man," p. 92:

"According to Fabre's observations, which Bergson accepts, theAmmophila stings its prey EXACTLY and UNERRINGLY in EACH of thenervous centres. The result is that the caterpillar is paralyzed,but not immediately killed, the advantage of this being that thelarva cannot be injured by any movement of the caterpillar, uponwhich the egg is deposited, and is provided with fresh meat whenthe time comes.

"Now Dr. and Mrs. Peckham have shown that the sting of the waspis NOT UNERRING, as Fabre alleges, that the number of stings isNOT CONSTANT, that sometimes the caterpillar is NOT PARALYZED,and sometimes it is KILLED OUTRIGHT, and that THE DIFFERENTCIRCUMSTANCES DO NOT APPARENTLY MAKE ANY DIFFERENCE TO THE LARVA,which is not injured by slight movements of the caterpillar, norby consuming food decomposed rather than fresh caterpillar."

This illustrates how love of the marvellous may mislead even socareful an observer as Fabre and so eminent a philosopher asBergson.

In the same chapter of Dr. Drever's book there are someinteresting examples of the mistakes made by instinct. I willquote one as a sample:

"The larva of the Lomechusa beetle eats the young of the ants, inwhose nest it is reared. Nevertheless, the ants tend theLomechusa larvae with the same care they bestow on their ownyoung. Not only so, but they apparently discover that the methodsof feeding, which suit their own larvae, would prove fatal to theguests, and accordingly they change their whole system ofnursing" (loc. cit., p. 106).

Semon ("Die Mneme," pp. 207-9) gives a good illustration of aninstinct growing wiser through experience. He relates how huntersattract stags by imitating the sounds of other members of theirspecies, male or female, but find that the older a stag becomesthe more difficult it is to deceive him, and the more accuratethe imitation has to be. The literature of instinct is vast, andillustrations might be multiplied indefinitely. The main pointsas regards instinct, which need to be emphasized as against thepopular conceptions of it, are:

(1) That instinct requires no prevision of the biological end which it serves;

(2) That instinct is only adapted to achieve this end in the usual circumstances of the animal in question, and has no moreprecision than is necessary for success AS A RULE;

(3) That processes initiated by instinct often come to beperformed better after experience;

(4) That instinct supplies the impulses to experimental movements which are required for the process of learning;

(5) That instincts in their nascent stages are easily modifiable, and capable of being attached to various sorts of objects.

All the above characteristics of instinct can be established bypurely external observation, except the fact that instinct doesnot require prevision. This, though not strictly capable of beingPROVED by observation, is irresistibly suggested by the mostobvious phenomena. Who can believe, for example, that a new-bornbaby is aware of the necessity of food for preserving life? Orthat insects, in laying eggs, are concerned for the preservationof their species? The essence of instinct, one might say, is thatit provides a mechanism for acting without foresight in a mannerwhich is usually advantageous biologically. It is partly for thisreason that it is so important to understand the fundamentalposition of instinct in prompting both animal and humanbehaviour.