CHAPTER XVI

KNOWLEDGE AND BELIEF

The Knowledge of the Truth and Its Sources: the Activity of the Senses and the Association of Presentations--Organs of Sense and Organs of Thought--Sense-Organs and their Specific Energy--Their Evolution--The Philosophy of Sensibility--Inestimable Value of the Senses--Limits of Sensitive Knowledge--Hypothesis and Faith--Theory and Faith--Essential Difference of Scientific (Natural) and Religious (Supernatural) Faith--Superstition of Savage and of Civilized Races--Confessions of Faith--Unsectarian Schools--The Faith of Our Fathers--Spiritism--Revelation

Every effort of genuine science makes for a knowledge of the truth. Our only real and valuable knowledge is a knowledge of nature itself, and consists of presentations which correspond to external things. We are incompetent, it is true, to penetrate into the innermost nature of this real world--the "thing in itself"--but impartial critical observation and comparison inform us that, in the normal action of the brain and the organs of sense, the impressions received by them from the outer world are the same in all rational men, and that in the normal function of the organs of thought certain presentations are formed which are everywhere the same. These presentations we call _true_, and we are convinced that their content corresponds to the knowable aspect of things. We _know_ that these facts are not imaginary, but real.

All knowledge of the truth depends on two different, but intimately connected, groups of human physiological functions: firstly, on the _sense-impressions_ of the object by means of sense-action, and, secondly, on the combination of these impressions by an association into _presentations_ in the subject. The instruments of sensation are the sense-organs (_sensilla_ or _aestheta_); the instruments which form and link together the presentations are the organs of thought (_phroneta_). The latter are part of the central, and the former part of the peripheral, nervous system--that important and elaborate system of organs in the higher animals which alone effects their entire psychic activity.

Man's sense-activity, which is the starting-point of all knowledge, has been slowly and gradually developed from that of his nearest mammal relatives, the primates. The sense-organs are of substantially the same construction throughout this highest animal group, and their function takes place always according to the same physical and chemical laws. They have had the same historical development in all cases. In the mammals, as in the case of all other animals, the _sensilla_ were originally parts of the skin; the sensitive cells of the epidermis are the sources of all the different sense-organs, which have acquired their specific energy by adaptation to different stimuli (light, heat, sound, chemical action, etc.). The rod-cells in the retina of the eye, the auditory cells in the cochlea of the ear, the olfactory cells in the nose, and the taste-cells on the tongue, are all originally derived from the simple, indifferent cells of the epidermis, which cover the entire surface of the body. This significant fact can be directly proved by observation of the embryonic development of man or any of the higher animals. And from this ontogenetic fact we confidently infer, in virtue of the great biogenetic law, the important phylogenetic proposition, that in the long historical evolution of our ancestors, likewise, the higher sense-organs with their specific energies were originally derived from the epidermis of lower animals, from a simple layer of cells which had no trace of such differentiated sensilla.

A particular importance attaches to the circumstance that different nerves are qualified to perceive different properties of the environment, and these only. The optic nerve accomplishes only the perception of light, the auditory nerve the perception of sound, the olfactory nerve the perception of smell, and so on. No matter what stimuli impinge on and irritate a given sense-organ, its reaction is always of the same character. From this specific energy of the sense-nerves, which was first fully appreciated by Johannes Müller, very erroneous inferences have been drawn, especially in favor of a dualistic and _à priori_ theory of knowledge. It has been affirmed that the brain, or the soul, only perceives a certain condition of the stimulated nerve, and that, consequently, no conclusion can be drawn from the process as to the existence and nature of the stimulating environment. Sceptical philosophy concluded that the very existence of an outer world is doubtful, and extreme idealism went on positively to deny it, contending that things only exist in our impressions of them.

In opposition to these erroneous views, we must recall the fact that the "specific energy" was not originally an innate, special quality of the various nerves, but it has arisen by adaptation to the particular activity of the epidermic cells in which they terminate. In harmony with the great law of "division of labor" the originally indifferent "sense-cells of the skin" undertook different tasks, one group of them taking over the stimulus of the light rays, another the impress of the sound waves, a third the chemical impulse of odorous substances, and so on. In the course of a very long period these external stimuli effected a gradual change in the physiological, and later in the morphological, properties of these parts of the epidermis, and there was a correlative modification of the sensitive nerves which conduct the impressions they receive to the brain. Selection improved, step by step, such particular modifications as proved to be useful, and thus eventually, in the course of many million years, created those wonderful instruments, the eye and the ear, which we prize so highly; their structure is so remarkably purposive that they might well lead to the erroneous assumption of a "creation on a preconceived design." The peculiar character of each sense-organ and its specific nerve has thus been gradually evolved by use and exercise--that is, by _adaptation_--and has then been transmitted by _heredity_ from generation to generation. Albrecht Rau has thoroughly established this view in his excellent work on _Sensation and Thought_, a physiological inquiry into the nature of the human understanding (1896). It points out the correct significance of Müller's law of specific sense-energies, adding searching investigations into their relation to the brain, and in the last chapter there is an able "philosophy of sensitivity" based on the ideas of Ludwig Feuerbach. I thoroughly agree with his convincing work.

Critical comparison of sense-action in man and the other vertebrates has brought to light a number of extremely important facts, the knowledge of which we owe to the penetrating research of the nineteenth century, especially of the second half of the century. This is particularly true of the two most elaborate "æsthetic" organs, the eye and the ear. They present a different and more complicated structure in the vertebrates than in the other animals, and have also a characteristic development in the embryo. This typical ontogenesis and structure of the sensilla of all the vertebrates is only explained by _heredity_ from a common ancestor. Within the vertebrate group, however, we find a great variety of structure in points of detail, and this is due to _adaptation_ to their manner of life on the part of the various species, to the increasing or diminishing use of various parts.

In respect of the structure of his sense-organs man is by no means the most perfect and most highly-developed vertebrate. The eye of the eagle is much keener, and can distinguish small objects at a distance much more clearly than the human eye. The hearing of many mammals, especially of the carnivora, ungulata, and rodentia of the desert, is much more sensitive than that of man, and perceives slight noises at a much greater distance; that may be seen at a glance by their large and very sensitive cochlea. Singing birds have attained a higher grade of development, even in respect of musical endowment, than the majority of men. The sense of smell is much more developed in most of the mammals, especially in the carnivora and the ungulata, than in man; if the dog could compare his own fine scent with that of man, he would look down on us with compassion. Even with regard to the lower senses--taste, sex-sense, touch, and temperature--man has by no means reached the highest stage in every respect.

We can naturally only pass judgment on the sensations which we ourselves experience. However, anatomy informs us of the presence in the bodies of many animals of other senses than those we are familiar with. Thus fishes and other lower aquatic vertebrates have peculiar sensilla in the skin which are in connection with special sense-nerves. On the right and left sides of the fish's body there is a long canal, branching into a number of smaller canals at the head. In this "mucous canal" there are nerves with numerous branches, the terminations of which are connected with peculiar nerve-aggregates. This extensive epidermic sense-organ probably serves for the perception of changes in the pressure, or in other properties, of the water. Some groups are distinguished by the possession of other peculiar sensilla, the meaning of which is still unknown to us.

But it is already clear from the above facts that our human sense-activity is limited, not only in quantity, but in quality also. We can thus only perceive with our senses, especially with the eye and the sense of touch, a part of the qualities of the objects in our environment. And even this partial perception is incomplete, in the sense that our organs are imperfect, and our sensory nerves, acting as interpreters, communicate to the brain only a translation of the impressions received.

However, this acknowledged imperfection of our senses should not prevent us from recognizing their instruments, and especially the eye, to be organs of the highest type; together with the thought-organs in the brain, they are nature's most valuable gift to man. Very truly does Albrecht Rau say: "All science is sensitive knowledge in the ultimate analysis; it does not deny, but interpret, the data of the senses. The senses are our first and best friends. Long before the mind is developed the senses tell man what he must do and avoid. He who makes a general disavowal of the senses in order to meet their dangers acts as thoughtlessly and as foolishly as the man who plucks out his eyes because they once fell on shameful things, or the man who cuts off his hand lest at any time it should reach out to the goods of his neighbor." Hence Feuerbach is quite right in calling all philosophies, religions, and systems which oppose the principle of sense-action not only erroneous, but really pernicious. Without the senses there is no knowledge--"_Nihil est in intellectu, quod non fuerit in sensu_," as Locke said. Twenty years ago I pointed out, in my chapter "On the Origin and Development of the Sense-Organs,"[32] the great service of Darwinism in giving us a profounder knowledge and a juster appreciation of the senses.

The thirst for knowledge of the educated mind is not contented with the defective acquaintance with the outer world which is obtained through our imperfect sense-organs. He endeavors to build up the sense-impressions which they have brought him into valuable knowledge. He transforms them into specific sense-perceptions in the sense-centres of the cortex of the brain, and combines them into presentations, by association, in the thought-centres. Finally, by a further concatenation of the groups of presentations he attains to connected knowledge. But this knowledge remains defective and unsatisfactory until the imagination supplements the inadequate power of combination of the intelligence, and, by the association of stored-up images, unites the isolated elements into a connected whole. Thus are produced new general presentative images, and these suffice to interpret the facts perceived and satisfy "reason's feeling of causality."

The presentations which fill up the gaps in our knowledge, or take its place, may be called, in a broad sense, "faith." That is what happens continually in daily life. When we are not sure about a thing we say, I believe it. In this sense we are compelled to make use of faith even in science itself; we conjecture or assume that a certain relation exists between two phenomena, though we do not know it for certain. If it is a question of a _cause_, we form a _hypothesis_; though in science only such hypotheses are admitted as lie within the sphere of human cognizance, and do not contradict known facts. Such hypotheses are, for instance--in physics the theory of the vibratory movement of ether, in chemistry the hypothesis of atoms and their affinity, in biology the theory of the molecular structure of living protoplasm, and so forth.

The explanation of a great number of connected phenomena by the assumption of a common cause is called a _theory_. Both in theory and hypothesis "faith" (in the scientific sense) is indispensable; for here again it is the imagination that fills up the gaps left by the intelligence in our knowledge of the connection of things. A theory, therefore, must always be regarded only as an approximation to the truth; it must be understood that it may be replaced in time by another and better-grounded theory. But, in spite of this admitted uncertainty, theory is indispensable for all true science; it elucidates facts by postulating a cause for them. The man who renounces theory altogether, and seeks to construct a pure science with certain facts alone (as often happens with wrong-headed representatives of our "exact sciences"), must give up the hope of any knowledge of causes, and, consequently, of the satisfaction of reason's demand for causality.

The theory of gravitation in astronomy (Newton), the nebular theory in cosmogony (Kant and Laplace), the principle of energy in physics (Meyer and Helmholtz), the atomic theory in chemistry (Dalton), the vibratory theory in optics (Huyghens), the cellular theory in histology (Schleiden and Schwann), and the theory of descent in biology (Lamarck and Darwin), are all important theories of the first rank; they explain a whole world of natural phenomena by the assumption of a common cause for all the several facts of their respective provinces, and by showing that all the phenomena thereof are inter-connected and controlled by laws which issue from this common cause. Yet the cause itself may remain obscure in character, or be merely a "provisional hypothesis." The "force of gravity" in the theory of gravitation and in cosmogony, "energy" itself in its relation to matter, the "ether" of optics and electricity, the "atom" of the chemist, the living "protoplasm" of histology, the "heredity" of the evolutionist--these and similar conceptions of other great theories may be regarded by a sceptical philosophy as "mere hypotheses" and the outcome of scientific "faith," yet they are indispensable for us, until they are replaced by better hypotheses.

The dogmas which are used for the explanation of phenomena in the various religions, and which go by the name of "faith" (in the narrower sense), are of a very different character from the forms of scientific faith we have enumerated. The two types, however--the "natural" faith of science and the "supernatural" faith of religion--are not infrequently confounded, so that we must point out their fundamental difference. Religious faith means always belief in a miracle, and as such is in hopeless contradiction with the natural faith of reason. In opposition to reason it postulates supernatural agencies, and, therefore, may be justly called superstition. The essential difference of this superstition from rational faith lies in the fact that it assumes supernatural forces and phenomena, which are unknown and inadmissible to science, and which are the outcome of illusion and fancy; moreover, superstition contradicts the well-known laws of nature, and is therefore _irrational_.

Owing to the great progress of ethnology during the century, we have learned a vast quantity of different kinds and practices of superstition, as they still survive in uncivilized races. When they are compared with each other and with the mythological notion of earlier ages, a manifold analogy is discovered, frequently a common origin, and eventually one simple source for them all. This is found in the "demand of causality in reason," in the search for an explanation of obscure phenomena by the discovery of a cause. That applies particularly to such phenomena as threaten us with danger and excite fear, like thunder and lightning, earthquakes, eclipses, etc. The demand for a causal explanation of such phenomena is found in uncivilized races of the lowest grade, transmitted from their primate ancestors by heredity. It is even found in many other vertebrates. When a dog barks at the full moon, or at a ringing bell, of which it sees the hammer moving, or at a flag that flutters in the breeze, it expresses not only fear, but also the mysterious impulse to learn the cause of the obscure phenomenon. The crude beginnings of religion among primitive races spring partly from this hereditary superstition of their primate ancestors, and partly from the worship of ancestors, from various emotional impulses, and from habits which have become traditional.

The religious notions of modern civilized peoples, which they esteem so highly, profess to be on a much higher level than the "crude superstition" of the savage; we are told of the great advance which civilization has made in sweeping it aside. That is a great mistake. Impartial comparison and analysis show that they only differ in their special "form of faith" and the outer shell of their creed. In the clear light of reason the refined faith of the most liberal ecclesiastical religion--inasmuch as it contradicts the known and inviolable laws of nature--is no less irrational a superstition than the crude spirit-faith of primitive fetichism on which it looks down with proud disdain.

And if, from this impartial stand-point, we take a critical glance at the kinds of faith that prevail to-day in civilized countries, we find them everywhere saturated with traditional superstition. The Christian belief in Creation, the Trinity, the Immaculate Conception, the Redemption, the Resurrection and Ascension of Christ, and so forth, is just as purely imaginative as the belief in the various dogmas of the Mohammedan, Mosaic, Buddhistic, and Brahmanic religions, and is just as incapable of reconciliation with a rational knowledge of nature. Each of these religions is for the sincere believer an indisputable truth, and each regards the other as heresy and damnable error. The more confidently a particular sect considers itself "the only ark of salvation," and the more ardently this conviction is cherished, the more zealously does it contend against all other sects and give rise to the fearful religious wars that form the saddest pages in the book of history. And all the time the unprejudiced "critique of pure reason" teaches us that all these different forms of faith are equally false and irrational, mere creatures of poetic fancy and uncritical tradition. Rational science must reject them all alike as the outcome of superstition.

The incalculable injury which irrational superstition has done to credulous humanity is conspicuously revealed in the ceaseless conflict of confessions of faith. Of all the wars which nations have waged against each other with fire and sword the religious wars have been the bloodiest; of all the forms of discord that have shattered the happiness of families and of individuals those that arise from religious differences are still the most painful. Think of the millions who have lost their lives in Christian persecutions, in the religious conflicts of Islam and of the Reformation, by the Inquisition, and under the charge of witchcraft. Or think of the still greater number of luckless men who, through religious differences, have been plunged into family troubles, have lost the esteem of their fellow-citizens and their position in the community, or have even been compelled to fly from their country. The official confession of faith becomes most pernicious of all when it is associated with the political aims of a modern state, and is enforced as "religious instruction" in our schools. The child's mind is thus early diverted from the pursuit of the truth and impregnated with superstition. Every friend of humanity should do all in his power to promote unsectarian schools as one of the most valuable institutions of the modern state.

The great value which is, none the less, still very widely attached to sectarian instruction is not only due to the compulsion of a reactionary state and its dependence on a dominant clericalism, but also to the weight of old traditions and "emotional cravings" of various kinds. One of the strongest of these is the devout reverence which is extended everywhere to sectarian tradition, to the "faith of our fathers." In thousands of stories and poems fidelity to it is extolled as a spiritual treasure and a sacred duty. Yet a little impartial study of the history of faith suffices to show the absurdity of the notion. The dominant evangelical faith of the second half of the nineteenth century is essentially different from that of the first half, and this again from that of the eighteenth century. The faith of the eighteenth century diverges considerably from the "faith of our fathers" of the seventeenth, and still more from that of the sixteenth, century. The Reformation, releasing enslaved reason from the tyranny of the popes, is naturally regarded by them as darkest heresy; but even the faith of the papacy itself had been completely transformed in the course of a century. And how different is the faith of the Christian from that of his heathen ancestors. Every man with some degree of independent thought frames a more or less personal religion for himself, which is always different from that of his fathers; it depends largely on the general condition of thought in his day. The further we go back in the history of civilization, the more clearly do we find this esteemed "faith of our fathers" to be an indefensible superstition which is undergoing continual transformation.

One of the most remarkable forms of superstition, which still takes a very active part in modern life, is _spiritism_. It is a surprising and a lamentable fact that millions of educated people are still dominated by this dreary superstition; even distinguished scientists are entangled in it. A number of spiritualist journals spread the faith far and wide, and our "superior circles" do not scruple to hold _séances_ in which "spirits" appear, rapping, writing, giving messages from "the beyond," and so on. It is a frequent boast of spiritists that even eminent men of science defend their superstition. In Germany, A. Zöllner and Fechner are quoted as instances; in England, Wallace and Crookes. The regrettable circumstance that physicists and biologists of such distinction have been led astray by spiritism is accounted for, partly by their excess of imagination and defect of critical faculty, and partly by the powerful influence of dogmas which a religious education imprinted on the brain in early youth. Moreover, it was precisely through the famous _séances_ at Leipzig, in which the physicists, Zöllner, Fechner, and Wilhelm Weber, were imposed on by the clever American conjuror, Slade, that the fraud of the latter was afterwards fully exposed; he was discovered to be a common impostor. In other cases, too, where the alleged marvels of spiritism have been thoroughly investigated, they have been traced to a more or less clever deception; the mediums (generally of the weaker sex) have been found to be either smart swindlers or nervous persons of abnormal irritability. Their supposed gift of "telepathy" (or "action at a distance of thought without material medium") has no more existence than the "voices" or the "groans" of spirits, etc. The vivid pictures which Carl du Prel, of Munich, and other spiritists give of their phenomena must be regarded as the outcome of a lively imagination, together with a lack of critical power and of knowledge of physiology.

The majority of religions have, in spite of their great differences, one common feature, which is, at the same time, one of their strongest supports in many quarters. They declare that they can elucidate the problem of existence, the solution of which is beyond the natural power of reason, by the supernatural way of revelation; from that they derive the authority of the dogmas which in the guise of "divine laws" control morality and the practical conduct of life. "Divine" inspirations of that kind form the basis of many myths and legends, the human origin of which is perfectly clear. It is true that the God who reveals himself does not always appear in human shape, but in thunder and lightning, storm and earthquake, fiery bush or menacing cloud. But the revelation which he is supposed to bring to the credulous children of men is always anthropomorphic; it invariably takes the form of a communication of ideas or commands which are formulated and expressed precisely as is done in the normal action of the human brain and larynx. In the Indian and Egyptian religions, in the mythologies of Greece and Rome, in the Old and the New Testaments, the gods think, talk, and act just as men do; the revelations, in which they are supposed to unveil for us the secrets of existence and the solution of the great world-enigma, are creations of the human imagination. The "truth" which the credulous discover in them is a human invention; the "childlike faith" in these irrational revelations is mere superstition.

The true revelation--that is, the true source of rational knowledge--is to be sought in nature alone. The rich heritage of truth which forms the most valuable part of human culture is derived exclusively from the experiences acquired in a searching study of nature, and from the rational conclusions which it has reached by the just association of these empirical presentations. Every intelligent man with normal brain and senses finds this true revelation in nature on impartial study, and thus frees himself from the superstition with which the "revelations" of religion had burdened him.