Popular Science Monthly/Volume 5/July 1874/The Development of Psychology I
THE progress of Psychology has been determined by agencies which may, with much precision, be discriminated as two sets of conflicting yet coöperating forces—those maintaining equilibrium, and those producing motion. This language would be justly condemned as mechanical if it in any degree presupposed the vulgar notion of force, as acting on visible masses of matter and causing sensible motion. But since vital, mental, and even social phenomena, as well as the oscillations of molecules and the ethereal undulations, are now alike interpreted in terms of mechanism, we may reasonably claim that the phraseology shall receive the greatest latitude of interpretation consistent with the admission of no mechanical assumptions. If, with more propriety, it be censured as scholastic, as raising mere observed uniformities into self-acting entities, it may be replied that the term force is scholastic only when used scholastically, that it has a true and unmistakable meaning as a generalization simply, and that progress of all kinds can be best described in the language of the science which has clothed the laws of the action of force with the greatest possible precision and certainty. Under these reservations, we use no mere metaphor in describing the development of Psychology as due to two sets of forces, which may be styled kinetical and statical respectively, according as their function has been to produce external change or to effect those internal readjustments which previous changes had rendered necessary.
The statical factor in psychological history is Theology. The mother of all the sciences, it gives birth to Psychology first of the sciences of mind; all the great problems, the discussion of which carries the science through its subsequent revolutions, are raised by it; and we may find that its perpetual function, of which it can never be discharged, is to recall attention from temporary physical solutions to the insoluble problems themselves.
The kinetical factor is constituted by the whole series of the physical sciences, though at any particular epoch it takes the character of the dominant science. Each stage in the development of Psychology corresponds to some stage in the evolution of the natural sciences; by each such transition has each psychological development been caused and conditioned; and the progress of Psychology in fundamental truth, and its more complete emancipation from Theology and Meta-physics, are to be measured by the degree in which physical methods, physical conceptions, and even physical metaphors, have been applied to the interpretation of the facts of mind.
The primitive savage, looking out upon the world, finds no God; gazing inward upon himself, perceives no Soul; and thinking of the origin of things, can conceive no Creation. His gods are parts of the world, not makers of it; such soul as he ascribes to himself is merely his own double, which perishes with him or soon after, or he has several souls; and the earth, as he sees it, was not made but hooked up from the bottom of the original sea. To the undiscriminating mind of the savage the Cosmos is accordingly all but homogeneous, with just the beginnings of "differentiation," and God, Man, and Nature, have yet to acquire an independent existence. There is still, therefore, no room for Psychology.
Plato gets several stages further than this. With him the Cosmos is a divine immortal being or animal, composed of a spherical rotatory body and a rational soul. The gods dwell in the peripheral or celestial regions, and men and the animals inhabit the lower or more central regions. The cranium of man is a little Cosmos, with an immortal rational soul, composed of the same materials as the cosmical soul, and moving with the like rotations. Within the body on which this cranium is placed are two inferior and mortal souls; one, the seat of courage, etc., in the chest; the other, the seat of appetite in the abdomen; both of them being rooted in the spinal marrow, which is continuous with the brain, and is the medium of the unity or communication of the three souls. In this semi-barbaric Cosmology we may note that the gods are still mixed up with the Cosmos, though the beginnings of separation are shown by their lodgment in a specific place; that they still want unity; and that there is yet no conception of nature. But we are here more concerned to observe that though the human soul is never actually separated from the body, i. e., is not yet detached from the Cosmos, and though it has the corporeal properties of extension and motion, body and soul, microcosmical and macrocosmical, are set sharply over against one another, and the first decided step toward their absolute separation is taken.
The metaphysical advance of Aristotle is immense. The three Platonic souls are merged in one, though the remains of the old idea are visible in the different attributes and distinct origin of the Nutritive, the Nutritive-sentient, and the Noëtic principles. But the Nutrient principle is the indispensable basis, without which neither of the others can exist, and the next higher principle, the Sentient, implies and contains the lower. In the investigations of the properties of these we have the beginnings of Psychology. It is not yet indeed an independent science, for the soul is still imperfectly extricated from the Cosmos—the Noëtic principle having its proper abode in the concave of heaven, and being only temporarily localized in the human body. The soul is still, as regards man, mortal, though as regards the Cosmos it is imperishable.
Between Aristotle and the thirteenth century the metaphysical evolution was slow, and the stages few and short. The idea of God as an independent existence received its first elaboration in the controversies of the Greek Fathers about the Trinity; was perhaps first sharply discriminated by Anselm; and was raised to the highest pitch of sublimation by the Deistic debates of the seventeenth century, with which the "return of the curve" begins. The idea of Nature, isolated alike from God and Man, emerged from the Italian pantheistic schools of the fifteenth century, to be decisively established with the foundation of Natural Philosophy. The idea of the Soul, with which we are here concerned, was the first of the three elements latent in the primitive homogeneous Cosmos to be completely "differentiated." Whether there was any intrinsic necessity in its earlier evolution; whether it was earlier developed because humanity itself and not merely the metaphysicians contributed to it; or whether it was solely the result of the working of the statical factor in the history of Psychology—the necessities of Theology; its first clear, though not complete, extrication may plausibly be placed as high up as the thirteenth century. As with the other two constitutive ideas, its emergence was the issue of a prolonged debate. No mediæval controversy made more noise while it lasted than the fierce war between the Averroists and the Schoolmen de unitate intellectus (concerning the oneness of the intellect). Averroës himself, the Arabian Hobbes, had been dead for half a century, but his doctrines had excited an extraordinary ferment among the younger and more speculative minds, and they reached the climax of their popularity just when the Scholastic Philosophy attained in Thomas Aquinas the culminating point in its history. East and West, Semitism and Aryanism, pantheistic absorption and political individualism, in the guise of Aristotle Arabized and Aristotle Christianized, met in final conflict, and the overthrow was, for the time at least, decisive. The theory of Averroës about the Soul was an imposing and picturesque development of the cosmical Psychology of Aristotle. The Nous of Aristotle was only temporarily localized in the body, and, after the death of the matter which it informed, returned to the grand region of Form, the Celestial Body. Averroës first severed the Nous from the Cosmos, unified it in humanity which it actualized, and made it eternal there. But it was only the common possession of the race through all time, and not particular to the individual; there were no souls, but only a single vast Soul, of which each generation was the perishable embodiment, but itself imperishable. Simple-minded, undoubting Thomas, with his eternal "Aristoteles dicit," "Aristoteles respondet," "Aristoteles habet" (Aristotle says, replies, has this), as if the question were to be thus settled, had no difficulty in showing that this was not, what the Averroists felt obliged to maintain, the doctrine of Aristotle."De Unitate Intellectûs," passim. But it was an advance upon that doctrine without which Aquinas's own unquestionable advance upon Averroës might never, or not so soon, have been made. While, however, Thomas successfully asserted against the Arabians the individuality of the soul, and against the older Aristotelians its substantial unity, there was still another step to be taken before its independence on all sides could be regarded as established, and the ground cleared for the science of Psychology. That step was taken by Descartes, in whom mankind may be said to have come to a consciousness of itself. His "Cogito, ergo sum" (I think; therefore I am), was not logical, but genetic. The force of the ergo (therefore), as Ferrier long ago pointed out, lay in the fact that the existence of Descartes as a self-conscious being—sum (I am)—was resultant upon the process described by the word cogito (I think)—the turning of the light of self-consciousness upon the thinking principle itself. We have but extended Ferrier's interpretation from the development of self-consciousness in the individual to the metaphysical evolution of the ego (me) in human history. Not till this had been accomplished, and the Mind made a separate individual existence as against God and Nature, was any independent science of Psychology possible. Observations and reasonings on Man, as on the Deity and the Creation, formed part of the "undifferentiated" mass of speculation on things in general called Cosmology or Theology, and latterly, in a mutilated condition, Metaphysics. Any mediæval cyclopædia will furnish illustrations.
Thomas Aquinas, a faithful representative of the frightened orthodoxy of the Middle Ages, unsuspectingly follows the course of Creation, well known to have happened as laid down in the Book of Genesis. After forty-four Quæstiones (Questions) on God (under whom he discusses the nature of ideas and the metaphysics of truth) and the Trinity, and thirty on the Angels, the Devils (here arises, naturally, a discussion on the nature of evil), and the seven days of creation and rest, Thomas arrives, by an obvious logical sequence, at the psychology of man. One quæstio (question) settles the essence of the soul, another the union of soul and body; three exhaust the powers of mind in general and special, and the intellectual powers; four expound appetite, sensuality, the will, and free-will; and, having in seven more disposed of the remaining faculties of the soul, including such small subjects as "the mode and order of intellection," Thomas is prepared to deal with the production of man's body, and then evidently with the production of woman's body. A witty journalist is reported to have said of an eminent living thinker, "God made the world in six days, and So-and-so wrote it down on the seventh;" but the entire Synthetic Philosophy might fall out of a corner of the "Summa Theologiæ" (Sum of Theology) and hardly be missed. Yet, arrogant as this encyclopedic comprehensiveness now seems, there was really nothing else to be done. Mathematics was the only one of the natural sciences which had succeeded in disengaging itself from theology; there was no social science, no independent science even of politics; there was no history other than ecclesiastical; and (what concerns us here) there was no science of man. Man was not yet a unit in the creation, and inquiries concerning him were properly included in Cosmology, which is pagan for Theology. "Naturam autem," says Thomas, "hominis considerare pertinet ad Theologum ex parte animæ" (It is the theologian's province to consider man's nature on the soul's side). The theologus kept hold of the nature of man till Descartes had emancipated him from his serfdom; but to him and his theological science—our statical factor—we may justly ascribe that first successful raising of the problem of human individuality which made possible, as we shall see, its establishment and utilization under the influence of the dynamical factor—physical science.
The fostering aid of Theology to Psychology does not, however, end when the latter is able to walk alone. All great questions subsequently raised, the settlement of which by physical methods marks each fresh stage, issue from the theological incunabula (cradle) where the science was reared. A history of the embryogeny of ideas would demonstrate that ideas which were afterward properly philosophical were at first purely theological. The idea of the infinite, at first negative, was made positive, through being made theological, by the Greek Fathers. Prof. Jevons believes that his "Law of Simplicity," though almost unnoticed in modern times, was known to Boëthius, and he adds:
"Ancient discussions concerning the doctrine of the Trinity drew more attention to subtle questions concerning the nature of unity and plurality than has ever since been given to them."
With greater emphasis, which, however, only exaggerates an important truth, it has been said that the doctrine of the Trinity is the "foundation of all the metaphysical thought and speculation of the ages after Gregory the Great." This will be sufficiently near the mark if the honor is shared with the dogma of Transubstantiation after, say, the "captivity" at Avignon. In more recent times, especially in Germany in the first half of the present century, the doctrine of the Incarnation has been the "motive" of various metaphysical developments.
In Psychology the final cause of Locke was theological; for the rise of an a priori philosophy in Herbert of Cherbury was theological, and it was to overthrow apriorism that Locke undertook his examination of the "original, certainty, and extent of human knowledge." Berkeley avowed that his motive, in investigating the nature of perception, was to provide a bulwark against the atheists. Hume is essentially theological, and in his "Inquiry concerning the Human Understanding," a section on Miracles stands side by side with one on the Idea of Necessary Connection. Reid wrote his "Essays on the Powers of the Human Mind" to refute Hume, and became, with this theological motive, the founder of Scotch psychology. Kant undertook his "Criticism of Pure Reason," and thus established a priori psychology, to show against Hume that the ideas of God, freedom, and immortality, could not be disproved by mere empirical reasoning. And the impulse which Hamilton, through Mansel, communicated to Psychology, by the new face he gave to the old problem of the Infinite, was a theological movement in its origin.
Under whatever name we give to it, under whatever form it may hereafter assume, Theology, the science of causes, essences, and origins, will play, as it has hitherto played, an important part in the development of the mental sciences, and especially of Psychology. When physical science is driving its ploughshare into untrodden regions till now only gazed down upon by the metaphysician in his balloon; when the speed of thought itself is measured; when the most complex effort of quantitative reasoning is proved to be fundamentallywith the simplest perception of relation; when the nature of intelligence is tracked upward in graduated sequence from the Radiata and Articulata to Newton and Shakespeare; and when the physical sides of all but the most subtle mental phenomena are being identified; the temptation is great to suppose that we are nearing the goal—that as so many laws of mind have been explained by physical laws, and so many facts interpreted in physical terms, the time is at hand, or at least will come, when the nature of causation, and of the substance of mind, and of the relation of phenomena to their source, and of that inscrutable source itself, will yield their secrets to the analysis of the inquirer armed with the weapons of physical science. Whatever power stands in the old place of Theology, which is dead—whether Metaphysics, if that be not dead also, or some "Unknowable" section of our compendiums of first principles—will show all such Comtist dreams to be vain, by eternally asking the unanswerable questions which it has been asking since the beginning of speculation. And each old question newly asked after each fresh advance of physical science tends to restore the equilibrium deranged by the operation of that dynamical factor, the history of the effects of which we will now, briefly sketch.
The application of physical methods to the phenomena of mind we believe to have originated in the fact that, outside the territory which (as we saw by the quotation from St. Thomas) was sacred to the theologus there was a sort of no-Man's land, which profane persons might enter into and possess. For Aquinas goes on: "Non autem ex parte corporis, nisi secundum habitudinem quam habet corpus ad animam." That is to say, while the anima intellectiva (intellective soul), which issues directly from the hand of God, is the exclusive province of the theologian, the anima sensitiva (sensitive soul), which is propagated in a physical manner, the passions, and the appetites, may be left to the uncowled cultivator of science as not requiring the help of divine inspiration. It was at any rate in this field that the foundations of inductive psychology were laid, and it was to the explanation of the simpler phenomena of sensibility that physical conceptions were first applied. The two greatest thinkers of the seventeenth century were almost simultaneously on the ground.
Descartes is not now remembered by his "Treatise on the Passions" (which was published within a year of Hobbes's work on "Human Nature"), and we only note it here as an early example of experimentalism in Psychology. We are more concerned to observe that his vindication of the immateriality of the thinking principle, and his clear perception of the unity of the mental aggregate, were almost contemporaneous with the "new geometry." We are not, indeed, solicitous with regard to Descartes to justify our thesis—that each advance in Psychology has been caused and conditioned by a corresponding and previous advance in physical science; for the enunciation of the Cartesian principle was less a fact in Psychology than the accomplishment of a stadium in the metaphysical evolution, which made Psychology possible. But, perhaps, it may not seem fanciful to mention that Cavalieri, "the generally reputed father of the new geometry," published in 1635 his Method of Indivisibles (which had been largely anticipated by Kepler), or to connect his leading principle—that a solid is generated out of an infinite number of surfaces placed one above another as their indivisible elements—with the effort to unite into a single substance, itself localized, the endless multiplicity of the mental manifestations. It is, at least, clear, that the application of physics to mind will follow the development of physics; and as physics has not yet advanced beyond the geometrical stage, as the period immediately preceding "Descartes's Meditations" was the epoch of a great geometrical advance, as we now know that in virtue of the consensus (mutual agreement) which governs all social phenomena, all the conceptions of any age are moulded in the same matrix, it seems not wholly imaginary to adduce the psychology of Descartes, who was himself an eminent geometer, as in some degree the result of the dominance of the earliest developed of the sciences.
Emerging from this doubtful region, we pass on to the terra firma (firm ground) of demonstrable fact. Hobbes was rather older than Descartes, but he had the advantage of delaying at least the publication of his speculations until another great scientific advance had been accomplished. We cannot state his antecedents better than in his own words:
"The beginning of astronomy, except observations, I think, is not to be derived from farther time than from Nicolaus Copernicus, who, in the age next preceding the present, revived the opinion of Pythagoras, Aristarchus, and Philolaus. After him, the doctrine of the motion of the earth being now received, and a difficult question thereupon arising concerning the descent of heavy bodies, Galileus in our time, striving with that difficulty, was the first that opened to us the gate of natural philosophy universal, which is the knowledge of the nature of motion. So that neither can the age of natural philosophy be reckoned higher than to him. Lastly, the science of man's body, the most profitable part of natural science, was first discovered with admirable sagacity by our countryman. Dr. Harvey, principal physician to King James and King Charles, in his books of the 'Motion of the Blood' and of the 'Generation of Living Creatures;' who is the only man I know that, conquering envy, hath established a new doctrine in his lifetime. Before these, there was nothing certain in natural philosophy, but every man's experiments to himself, and the natural histories, if they may be called certain, that are no certainer than civil histories. But since these, astronomy and natural philosophy have, for so little time, been extraordinarily advanced by Joannes Keplerus, Petrus Gassendus, and Marinus Mersennus; and the science of human bodies in special by the wit and industry of physicians, the only true natural philosophers, especially of our most learned men of the College of Physicians in London. Natural Philosophy is therefore but young; but Civil Philosophy yet much younger, as being no older (I say it provoked, and that my detractors may know how little they have wrought upon me) than my own book, 'De Cive' (the Citizen.)"
The application of all this to the psychological philosophy of Hobbes is so patent as hardly to need elucidation in detail. Like his contemporary Descartes, Hobbes was extremely jealous of his independence, and, what was of less consequence, his originality; and one may even now hear, not without surprise and otherwise, the unlucky epigram which makes him say that, if he had read as many books as other people, he would have been as ignorant as they. Hobbes had read a great deal more than he deemed it prudent to admit, and if he had read more still the good effect of it would not have been doubtful. But, like the Greeks in the time of Sophocles, he had an advantage which would have made up for any deficiency of literary acquisition. He lived in an atmosphere heavy with ideas, and at a time when epistolary communication performed the functions very much which scientific journals now fulfill. Hobbes does not appear to have corresponded with Descartes, but he was in constant intercourse, by letter, with Mersenne, who acted as the intermediary between the two philosophers. And, as philosophers then concerned themselves with the whole range of the sciences, there was hardly a speculation stirring the European mind that need have escaped the notice of even a thinker somewhat out of the main lines of communication. Hobbes was, moreover, a traveller, had lived much on the Continent, and had possibly met Galileo at Pisa. It was under the influence of these two men, or rather of the methods they represented—Descartes and mathematics, Galileo and the laws of motion—that Hobbes proceeded to work out his philosophy. In the language of a distinguished professor, to whom we look for an exhaustive account of Hobbes's relations to the science of his time, "he set about reducing all his thoughts into the unity of a system, whose central idea was this of motion, and whose guiding principles were those of mathematical deduction." "His great postulate," says the same writer, "is motion or mutation," and he makes copious use of it within the sphere to which Aquinas banished the experimental psychologist, and a little beyond. His explanation of sensation is wholly mechanical. The crass materialism with which he set out may have had something to do with his trenchant rejection of the audible, visible, and intelligible species of the Schoolmen, but the hypothesis which replaced them betrays its own origin. "The apparition of light," he says, "is really nothing but motion within." This thesis is more elaborately developed in a passage which we quote at length, as it appears to contain an anticipation of the undulatory theory of light and heat:
"From all lucid, shining, and illuminate bodies, there is a motion produced to the eye, and, through the eye, to the optic nerve, and so into the brain, by which that apparition of light and color is effected.... First, it is evident that the fire... worketh by motion equally every way.... And further, that that motion, whereby the fire worketh, is dilation, and contraction of itself alternately... is manifest also by experience. From such motion in the fire must needs arise a rejection or casting from itself of that part of the medium which is contiguous to it, whereby that part also rejecteth the next, and so successively one part beateth back another to the very eye," and so from the eye to the optic nerve, and from that to the brain.
This postulate of motion, applied in this thorough-going manner, led Hobbes to a great discovery in the psychology of sensation. He clearly demonstrated that the secondary qualities of body are purely subjective, and his language is almost strong enough to lead us to believe that he would have gone a long way with Berkeley. For he claims to have proved that "as in vision, so also in conceptions that arise from the other senses, the subject of their inherence is not the object but the sentient." If the word "conceptions" be interpreted according to a definition previously laid down in the same treatise, in which the "images produced by things" are described as conceptions, imaginations, ideas, knowledge, it should seem that he might have applied the analysis to the primary qualities as well, had the two sets of properties been as sharply contrasted as now, instead of being first discriminated by Descartes and Hobbes. The same conception (motion) is used to explain the feelings, which, when pleasurable, are the result of the vital motion being "helped" by the motions which, having produced conceptions in the head, afterward proceed to the heart. But external objects not only "cause conceptions, and conceptions appetite, and fear;" as the latter are "the first unperceived beginnings of our actions," and as in a state of doubt, appetite and fear rapidly succeed one another, "this alternate succession of appetite and fear.... is that we call deliberation." As all Hobbes's successors of the same school have followed him in thus ignoring the ego, it may be inferred that every system of experimental psychology is self-condemned to incompleteness, and that no system can cover the whole of the ground which does not make what can only be called metaphysical assumptions.
The psychological advances made by Hobbes were then—that he helped to banish the imaginary entities of the Schoolmen, and substituted for them hypotheses that implied at least veræ causæ (true causes); that he replaced the method of deduction from assumed principles by that of observation (which was not yet, however, that of introspection), and thus founded the inductive philosophy of the mind; and that by his summary rejection of the common metaphysical assumptions, and his patient building up on an independent foundation, he decisively separated psychology from the metaphysics in which it was enmeshed.
If the psychology of Hobbes bears evident marks of the daring, speculative character of contemporary physical science, that of Locke witnesses to the change in the tone and spirit of inquiry. If the keyword to Hobbes is Galileo, that to Locke is Sydenham. Locke and Sydenham were both surgeons, were friends, and were of kindred cautious temperament; and the pacific revolution which Sydenham wrought in medicine has been described in language that, with the necessary change of terms, might word for word be applied to the great psychological advance initiated by Locke. A competent writer describes Sydenham as being—
"most careful to exclude the prevailing theories from affecting his study of the facts of disease: he followed the inductive method which his countryman, Bacon, had just completed, and under the guidance of his friend John Locke, himself a surgeon, he applied it to the investigation of disease with splendid success. The laws ruling the prevalence of epidemics were elucidated, and new and old diseases described with an accuracy and graphic coloring which have ever since remained unrivaled. The treatment of disease Sydenham found lamentably uncertain from want of any fixed principle, and from the countless remedies prescribed mainly in accordance with a capricious fashion. In place of this, he left therapeutics an art ordered by the principle of aiding Nature, and observing the indications afforded by morbid processes themselves....
Bacon had. justly reproached the physicians of his time for their neglect to make records of the cases of their patients.... Sydenham.... by his bedside study again brought it into favor." And finally, "he found English medicine reduced to the lowest state of empiricism—he raised it once more to the dignity of a science of observation."
The disposition in which Locke entered on his inquiry was certainly "to exclude prevailing theories," for he has himself recorded that his Essay originated in a conviction that, before advancing to abstruse problems, "it was necessary to examine our own abilities, and see what objects our understandings were or were not fitted to deal with." His method of induction was truly Baconian: he approached the subject without any clear design, proceeded without a plan, and attained such results as can be so reached. But the "laws ruling the" formation of ideas were elucidated, and mixed and simple modes "described with an accuracy" and in one or two cases with "a graphic coloring" which have not been greatly surpassed. The philosophy of the mind he found an untrodden jungle, with a few bridle-paths in the directions marked "Sense," "Appetite," etc.; he cut a highway through the part where the bush was thickest—the region of ideas. The a priori method was in favor, and "bedside study" of the human patient out of fashion; the a priori method he did not indeed kill, but he left it to die a lingering death; and though to Hobbes belongs the honor of introducing the experimental method into Psychology, it may be truly said of Locke that he "raised it to the dignity of a science of observation." And just as Sydenham, follower of Hippocrates as he was, attributed a number of diseases to morbid fermentation in the humors, so Locke, in spite of his antischolasticism, could still assign the motion of the "animal spirits" as a "natural cause" of certain ideas. The defects and the merits, in truth, of Locke's procedure were equally those of the physical science of the age. The patient observation of which Sydenham set the example gave rise to the first discriminative account—we can hardly call it analysis—of the proximate origin and more obvious constituents of our ideas. To the same causes and doubtless also to the impulse of conquest in unexplored regions which the post-mediæval world owed to Bacon, we may ascribe it that Locke's "Essay," as he named it, "inquiry," as he described it, was the first comprehensive survey of mental phenomena; while the small part which hypothesis and theory play in his investigation, his incomplete statement of mental causation of all kinds, his bare discovery of association as producing a few obvious compounds, were clearly due to the unspeculative character of the contemporary science to the influence of which he was most exposed.
Berkeley's most notable contribution to philosophy belongs rather to the metaphysics, than to the psychology, of sensation; and his less disputed discovery of the acquired nature of our perceptions of distance we may pass over with the remark that, if the genesis of it could be traced, it would probably be found to have derived its impulse from that "century of inventions" which witnessed Snell's discovery of the law of refraction in 1624, Newton's discoveries in the composition of light in 1674, Huyghens's proof of the polarization of light about 1692, and the explanation of the structure of the eye by Petit in 1700. The conjunction will seem more than a coincidence if it is added that Berkeley's "Theory of Vision," which appeared in 1709, was preceded by Newton's "Optics," in 1705.
The next great advance of Psychology combined, in principle, the advances made by both Hobbes and Locke. As Hobbes had incorporated the conceptions of physical science, and Locke had adopted its methods, we find Hartley professing to follow the "method of analysis and synthesis recommended and followed by Sir Isaac Newton," and appropriating from the "Principia" the hypothesis of vibrations by which he explained sensation:
"My chief design in the following chapter is, briefly, to explain, establish, and apply the doctrines of vibrations and association. The first of these doctrines is taken from the hints concerning the performance of sensation and motion, which Sir Isaac Newton has given at the end of his 'Principia,' and in the questions annexed to his 'Optics;' the last from what Mr. Locke and other ingenious persons since his time have delivered concerning the influence of association over our opinions and affections, and its use in explaining those things in an accurate and precise way, which are commonly referred to the power of habit and custom, in a general and indeterminate one.... One may expect that vibrations should infer associations as their effect, and association point to vibrations as its cause."
It may seem somewhat bold in Hartley, whose name has almost passed into a by-word as that of an hypothesis-maker, to shelter himself under the ægis of Newton, who  Psychology had reached in Hartley's time, as Natural Philosophy in Newton's time, the stage when the mere generalization of observed uniformities is no longer sufficient to cope with the accumulated multitude of ascertained facts, and when some comprehensive hypothesis is required which shall connect the empirical generalizations of one science with the ultimate laws of Nature and the principles of all the sciences. Newton's force of gravity and Hartley's theory of vibrations were such hypotheses. But, besides the intrinsic difference between them residing in the fact that the one could be proved, and the other, at best, only made probable, there was the further contrariety, which explains their very different success, that the Newtonian conception was the complement of a slow development. The first natural philosophers, down even to Kepler and Galileo, had contented themselves with studying effects, e. g., the orbits described by the heavenly bodies, and the period of their revolutions. But, with the decay of the scholastic metaphysics, which was also physics, a new idea began to stir the minds of men—that of force. It is said to have been conceived by Nicolas of Cusa; it found tortuous expression in Descartes's Vortices; and, specialized as governing gravitation, it was perhaps first dimly seen by Gilbert little less than a century before Newton, was asserted by Kepler nine years later (1609), and in 1674 was stated by Hooke with remarkable clearness and accuracy—all before Newton had thrown out any hint of his sublime discovery. Hartley's hypothesis, on the other hand, was a chance shot, a private guess, and was no matured result of previous theorizing. It accordingly passed into the limbo to which Nature consigns her mistakes; but the gain to Psychology was, though not equally great, of fundamentally the same kind as the gain to Natural Philosophy from the establishment of the law of gravitation. The idea of force subsumed that of law, the conception of causation superseded those of sequence and conjunction; and the basis for an explanation of the phenomena of mind was for the first time sought outside the limits of these phenomena. Hartley was unsuccessful, but the mere attempt has been as a light on high to guide the uncertain steps of later inquirers, and has at last led to the physical syntheses of our own day.—"hypotheses non fingo." But, as is observed by Prof. Stanley Jevons, "the greater part of the 'Principia' is purely hypothetical, endless varieties of causes and laws being imagined which have no counterpart in Nature."
Even a false, or at least a partially true, theory has the advantage of making possible a reasoned arrangement of the facts, as well as the acquisition of more. To Hartley this hypothesis of vibrations gave strength of wing to sweep the entire field of Psychology, and we accordingly find that his was the first systematic effort to explain the phenomena of mind by the law of association.
A very great advance in Psychology was made by James Mill, and it was initiated by Chemistry. During the first ten years of the nineteenth century Chemistry was revolutionized. In 1800 Nicholson and Carlisle decomposed water by means of the Voltaic pile, and enabled Davy in 1806 to make the generalizations which founded electrochemistry. The decomposition of potassa, soda, and other bodies of the same kind, soon followed. Beginning with hydrochloric acid in 1809, the discovery of the various hydracids was made. And in 1803-'04 a great synthetic addition was made to the analytic gains; Dalton's law of chemical combination was established." The influence of these brilliant discoveries upon the thought of the age was not doubtful. The literature of the day was drenched, with metaphors taken from the dominant science. Fashion, after a long interval, once more patronized Nature, and the "bottle-and-squirt mania" spread. Experimentalism in Psychology was still under a cloud, from the discredit which had attached to the premature theorizing of Hartley. But in the early part of the century, Dr. Thomas Brown had gained a hearing, under cover of the respectable orthodoxy of the Scotch universities, for speculations thickly sown with revolutionary germs. One of his pupils was James Mill, and in 1829 that resolute and thorough-going, if narrow and aggressive, thinker published the treatise which marked the turn of the tide. Deriving his inspiration from the neglected work of Hartley, gathering up the hints freely scattered in Brown's lectures, and imbued with the spirit of the prevailing chemistry, he set about constructing a new science of mind, of which the physics should not be obsolete, and which should push the analysis of the accepted metaphysical mysteries to the farthest possible limit. He obeyed the double analytic and synthetic movement in contemporary chemical investigation. As specimens of his analytical advance, we may point to his further resolution of the apparently simple ideas of hardness and extension, which had been begun by Hartley and continued by Darwin. But, as better illustrating the dynamical influence of physical science, we prefer to lay emphasis on what may, as it appears to us, be justly styled his synthetical contribution to Psychology. This was his conception, applied to the whole range of mental phenomena, of the chemical nature of association. Quite to realize the new shape which the welding mental power took in his hands, we must glance back at its history. It is comparatively young. Hobbes knew nothing of it: his "synthesis," by which things are "constructed or generated," is purely geometrical, and with him association is mere sequence. Locke's advance on this is clear, though inconsiderable: he speaks of the "tying together of ideas," and describes certain ideas as appearing in "gangs, always inseparable," but he regards "mixed modes" as made by men voluntarily with a view to communication. Hartley, according to Mr. J. S. Mill, had reached the stage we have above stated as only attained by James Mill:
"It was reserved for Hartley to show that mental phenomena, joined together by association, may form a still more intimate, and as it were chemical union; . . . the compound having all the appearance of a phenomenon sui generis, as simple and elementary as the ingredients, and with properties different from any of them."
This is far too strongly stated. That the union of the associated mental elements as conceived by Hartley was more intimate than their mode of conjunction as conceived by Locke, or their rigidity of sequence as imagined by Hobbes, is unquestionable; but how Mr. Mill could describe that union as chemical, and as analogous to the compound formed like water, by hydrogen and oxygen, is inexplicable if it be remembered that the composition of water was not discovered by Cavendish till 1784—thirty-five years after the appearance of the "Observations"—and that Chemistry only passed from the metaphysical to the positive stage with the deposition of phlogiston by Priestley and Lavoisier in the last quarter of the century. The following quotations from Hartley himself will confirm this a priori argument by showing the real nature of association as figured by him:
"Upon the whole, it may appear to the Reader, that the simple Ideas of Sensation must into Clusters and Combinations, by Association; and that each of these will, at last, coalesce into one Complex Idea, by the Approach and Commixture of the several compounding Parts."
No chemist would describe chemical union as "coalescence," or speak of the new substance produced by the operation of affinity as made up of "clusters and combinations" by the "approach and commixture" of parts. As appears still more clearly when Hartley proceeds to explain and illustrate this "coalescence," he had in his mind, as the physical type of his conception, no more "intimate union" than that combination of different kinds of matter called solution:
"If the Number of simple Ideas which compose the complex one be very great, it may happen that the complex Idea shall not appear to bear any relation to these its compounding Parts, nor to the external Senses upon which the original Sensations, which gave Birth to the compounding Ideas, were impressed. The Reason of this is, that each single Idea is overpowered by the Sum of all the rest, as soon as they are all intimately united together. Thus, in very compound Medicines, the several Tastes and Flavors of the separate Ingredients are lost and overpowered by the complex one of the whole Mass: so that this has a Taste and Flavor of its own, which appears to be simple and original, and like that of a natural Body."
We should be disposed to describe Hartley's view of mental composition as bearing a similar relation to James Mill's synthesis as Newton's composition of light to Goethe's theory of colors—as implying some species of union closer than the mechanical and less binding than the chemical. Thomas Brown clearly stated the law-as chemically conceived, in one of his introductory lectures. In mere statement James Mill's exposition is no advance upon Brown's, but the law took enormous extensions in his hands, and was applied to the senses, the feelings, memory, classification, language, ratiocination, the will, belief, etc. Something has been added to his synthesis, and a little has been taken from it, but he appears to have made as much as could be made out of the bare laws of association, unextended to the rest of the animal kingdom, and confined to the existing generation. His conception of the indissolubleness of certain associations, in particular, preluded the elucidation of their organic character as resulting from the intercourse of the mind with its environment.—Westminster Review.