In so far as the faculty of understanding is a physical object, the knowledge of its nature is a matter of physical science. But in so far as we understand all things by the help of this faculty, the science of understanding becomes metaphysics. Inasmuch as the scientific analysis of reason reverses the current conception of its nature, this specific understanding necessarily reverses our entire world philosophy. With the understanding of the nature of reason, we arrive at the long sought understanding of the "nature of things."

We wish to know, understand, conceive, recognize all things in their very nature, not in their outward appearance. Science seeks to understand the nature of things, or their true essence, by means of their manifestations. Every thing has its own special nature, and this nature is not seen, or felt, or heard, but solely perceived by the faculty of thought. This faculty explores the nature of all things just as the eye explores all that is visible in things. Just as the nature of sight is understood by the theory of vision, so the nature of things in general is understood by the theory of understanding.

It is true that it sounds contradictory to say that the nature of a thing does not appear to the eye, but to the faculty of thought, and at the same time to imply that the opposite of appearance, nature, should appear. But we here refer to the nature of a thing as a phenomenon in the same way in which we referred to the mind as a perception of the senses, and we shall demonstrate further on that every being is a phenomenon, and every phenomenon is more or less of an essential thing.

We have seen that the faculty of thought requires for its vital activity an object, or raw material. The effect of reasoning is seen in science, no matter whether we understand the term science in its narrow classical sense or in its broadest meaning of any kind of knowledge. The phenomena of sense perception constitute the general object or material of science. Sense perceptions arise from infinite circulation of matter. The universe and all things in it consist of transformations of matter which take place simultaneously and consecutively in space and time. The universe is in every place and at any time itself, new, and present for the first time. It arises and passes away, passes and arises under our very hands. Nothing remains the same, only the infinite change is constant, and even the change varies. Every particle of time and space brings new changes. It is true that the materialist believes in the permanency, eternity, indestructibility of matter. He teaches us that not the smallest particle of matter has ever been lost in the world, that matter simply changes its forms eternally, but that its nature lasts indestructibly through all eternity. And yet, in spite of all distinctions between matter itself and its perishable form, the materialist is on the other hand more inclined than any one else to dwell on the identity of matter and its forms. Inasmuch as the materialist speaks ironically of formless matter and matterless forms, in the same breath with perishable forms of imperishable matter, it is plain that materialism is not informed any more than idealism as to the relation of content to form, of a phenomenon to the essential nature of its subject. Where do we find such eternal, imperishable, formless matter? In the world of sense perceptions we never meet anything but forms of perishable matter. It is true that there is matter everywhere. Wherever anything passes away, something new instantly arises. But nowhere has any homogeneous, unchangeable matter enduring without any form, ever been discovered. Even a chemically indivisible element is only a relative unit in its actual existence, and in extension of time as well as in extension through space it varies simultaneously and consecutively as much as any organic individual which also changes only its concrete forms, but remains the same in its general nature from beginning to end. My body changes continually its fleshy tissue, bones, and every other particle belonging to it, and yet it always remains the same. What constitutes, then, this body which is distinguished from its transient form? It is the sum total, in a generalized way, of all its varied concrete forms. Eternal and imperishable matter exists in reality only as the sum total of its perishable forms. The statement that matter is imperishable cannot mean anything but that there will always and everywhere be matter. It is just as true to say that matter is imperishable and merely changes its forms, as it is to say that matter exists only in its changing forms, that it is matter which changes and that only the change is eternal. The terms "changeable matter" and "material change" are after all only different expressions for the same thing.

In the practical world of sense perceptions, there is nothing permanent, nothing homogeneous, nothing beyond nature, nothing like a "thing itself." Everything is changing, passing, phantomlike, so to say. One phantom is chased by another. "Nevertheless," says Kant, "things are also something in themselves," for otherwise we should have the absurd contradiction that there could be phenomena without things that produce them. But no! A phenomena is no more and no less different from the thing which produces it than the stretch of a twenty-mile road is different from the road itself. Or we may distinguish between a knife and its blade and handle, but we know that there would be no knife if there were no blade and no handle. The essential nature of the universe is change. Phenomena appear, that is all.

The contradiction between the "thing itself," or its essence, and its outward appearance is fully solved by a complete critique of reason which arrives at the understanding that the human faculty of thought may generalize any number of varied sense perceptions under one uniform point of view, by singling out the general and equivalent forms and thus regarding everything it may meet as a concrete part of one and the same whole.

In other words, the relative and transient forms perceived by our senses serve as raw material for our brain activity, which abstracts the general likeness out of the concrete forms and systematizes or classifies them for our consciousness. The infinite variety of sense perceptions passes in review before our subjective mind, and it constructs out of the multiplicity the unity, out of the parts the whole, out of the phenomena the essential nature, out of the perishable the imperishable, out of the attributes the subject. The essence, the nature of things, the "thing itself" is an ideal, a spiritual conception. Consciousness knows how to make sums out of different units. It can take any number of units for its sums. The entire multiplicity of the universe is theoretically conceived as one unit. On the other hand, every abstract sum consists in reality of an infinite number of sense perceptions. Where do we find any indivisible unit outside of our abstract conceptions? Two halves, four fourths, eight eighths, or an infinite number of separate parts form the raw material out of which the mind fashions the mathematical unit. This book, its leaves, its letters, or their parts, are they units? Where do I begin, where do I stop? In the same way, I may call a library with many volumes, a house, a farm, and finally the whole universe, a unit. Is not everything a part, is not every part a thing? Is the color of a leaf less of a thing than that leaf itself? Perhaps some would call the color simply an attribute and the leaf its substance, because there might be a leaf without color, but no color without a leaf. But as surely as we exhaust a heap of sand by scattering it, just as surely do we remove all the substance of a leaf when we take away its attributes one after the other. Color is only the sum of reactions of leaf, light, and eye, and so is all the rest of the matter of a leaf an aggregate of interactions. In the same way in which our reason deprives a leaf of its color attributes and sets it apart as a "thing itself," may we continue to deprive that leaf of all its other attributes, and in so doing we finally take away everything that makes the leaf. Color is in its nature no less a substance than the leaf itself, and the leaf is no less an attribute than its color. As the color is an attribute of a leaf, so a leaf is an attribute of a tree, a tree an attribute of the earth, the earth an attribute of the universe. The universe is the substance, substance in general, and all other substances are but its attributes. And this world-substance reveals the fact that the nature of things, the "thing itself" as distinguished from its manifestations, is only a concept of the mind.

In its universal search from the attribute to the substance, from the relative to the absolute, from the appearance of things to the true things, the mind finally arrives at the understanding that the substance is nothing but a sum of attributes collected by brain activity, and that the mind itself, or reason, is a substantial being which creates abstract mental units out of a multitude of sense perceptions and conceives of the universe as an absolute whole, as an independent "thing itself," by adding all its transient manifestations. In turning away full of dissatisfaction from attributes, searching restlessly after the substance, throwing aside phenomena, and forever groping for truth, for the nature of things, for the "thing itself," and in finally realizing that this substantial truth is merely the sum of all socalled untruths, the totality of all phenomena, the mind proves itself to be the creator of the abstract concept of substance. But it did not create this concept out of nothing. On the contrary, it generated the concept of a world substance out of attributes, it derived truth out of manifestations of things.

The idealist conception that there is an abstract nature behind phenomena which materialises itself in them, is refuted by the understanding that this hidden nature does not dwell in the world outside of the human mind, but in the brain of man. But since the brain differentiates between phenomena and their nature, between the concrete and the general, only by means of sense perception, it cannot be denied that the distinction between phenomena and their nature is well founded; only the essential nature of things is not found back of phenomena, but by means of phenomena. This nature is materially existent and our faculty of thought is a real and natural one.

It is true of spiritual things as well as of physical ones, in fact it is true of all things, metaphysically speaking, that they are what they are, not "in themselves," not in their abstract nature, but in contact with other things, in reality. In this sense one might say that things are not what they seem, but manifest themselves because they are existent, and they manifest themselves in as many different ways as there are other things with which they enter into relations of time and space. But the statement that things are not what they seem requires, in order to be rightly understood, the modification that whatever manifests itself, exists in nature, and its existence is limited by its manifestations. "We cannot perceive heat itself," says a book on physics written by Professor Koppe, "we merely conclude from its manifestations that it is present in nature." Thus reasons a naturalist who seeks to understand a thing by practical and diligent study of its manifestations, but who seeks refuge in the speculative belief in a hidden "thing itself" whenever a lack of understanding of the fundamentals of logic embarrasses him. We, on the contrary, conclude that there is no such thing as "heat itself," since it cannot be found, in nature, and we conceive of heat as effects of matter which the human brain translated into the conception of "heat itself." Because science was, perhaps, as yet unable to analyse this conception, the professor says we cannot perceive the natural object which gives rise to this conception. "Heat itself" is simply composed of the sum total of its manifold effects, and there is nothing else to it. The faculty of thought generalizes this variety of effects under the concept of heat in general. The analysis of this conception, the discovery of the general character of the various manifestations of heat, is the function of inductive science. But the conception of heat separated from its effects is a speculative idea, similar to Lichtenberg's knife without handle and blade.

The faculty of thought in touch with sense perceptions produces the nature of things. But it produces them no more independently of things outside than do the eye, the ear, or any other sense of man. It is not the "things themselves" which we see or feel, but their effects on our eyes, hands, etc. The faculty of reason to generalize different perceptions of the eye permits us to distinguish between concrete sights and sight in general. The faculty of thought conceives of any concrete sight as an object of sight in general. It furthermore distinguishes between subjective and objective sight perceptions, the latter being sights which are visible not alone to the individual eye, but to eyesight in general. Even the visions of a spiritualist, or such subjective impressions as forked lightning, circles of fire, caused by excited blood of closed eyes, serve as objects for the critical consciousness. A glittering object revealed by bright sunlight miles away is no more and no less tangible in substance, no more and no less true, than any optical illusion. A man whose ear is tingling hears something, though it is not the tinkling of bells. Every sense perception is an object, and every object is a sense perception. The object of any subjective mind is a passing manifestation, and every objective perception is but a perishable subject. The object of observation may exist in a more tangible, less approachable, more stable, or more general form, but it is not a "thing itself." It may be perceived not alone by my eyes, but also by those of others, not by the eyes, but also by the feeling, the hearing, the taste, etc. And it may be noticed not alone by men, but also by other objects. But nevertheless it appears only as a manifestation, it is different in different places, it is not today what it is tomorrow. Every existence is relative, in touch with other things, and entering into different relations of time and space with them.

Every sense perception is an actual and natural object. Truth exists in the form of natural phenomena, and whatever is, is true. Substance and attribute are only terms for certain relations. They are not contradictions, and, as a matter of fact, all contradictions disappear before our faculty of generalization and differentiation. For this faculty reconciles all contradictions by finding a general quality in all differences. Existence, or universal truth, is the general object, the raw material, of the faculty of thought. This material is of the utmost variety and supplied by the senses. The senses reveal to us the substance of the universe in the forms of concrete qualities, in other words, the nature of perceptible matter is revealed to the faculty of thought through a variety of concrete forms. It is not perceived as a general essence, but only through interdependent phenomena. Out of the interdependence of the sense perceptions with our faculty of thought there arise quantities, general concepts, things, true perceptions, or understood truths.

Essence and truth are two terms for the same thing. Truth, or the essence and nature of things, is a theoretical concept. As we have seen, we receive impressions of things in two ways, viz., a sense impression and a mental impression, the one practical, the other theoretical. Practice furnishes us with the sense impression, theory with the mental nature of things.

Practice is the premise of theory, sense perception the premise of the nature which is also called the truth. The same truth manifests itself in practice either simultaneously or consecutively in the same place or in different places. It exists theoretically as a homogeneous conception.

Practice, phenomena, sense perceptions, are absolute qualities, that is to say they have no quantitative limitation, they are not restricted by time or space. They are absolute and infinite qualities. The qualities of a thing are as infinite as its parts. On the other hand, the work of the faculty of thought, of theory, creates at will an infinite number of quantities, and it conceives every quality of sense perceptions in the form of quantities, as the essential nature of things, as truths. Every conception has a quality of some sense perception for its object. Every object can be conceived by the faculty of thought only as a quantitative unit, as true nature, as truth.

The faculty of thought produces in contact with sense perceptions that which manifests itself as true nature, as a general truth. A primitive concept accomplishes this at first only instinctively, while a scientific concept is a conscious and voluntary repetition of this primitive act. Scientific understanding wanting to know an object, such as for instance heat, is not hunting after the phenomena themselves. It does not aim to see or hear how heat melts iron or wax, how it benefits in one case or injures in another, how it makes eggs solid or ice liquid, nor does it concern itself with the difference between the heat of an animal, of the sun, or of a stove. All these things are from the point of view of the faculty of understanding, only effects, phenomena, qualities. It desires to get at the essence, the true nature of things, it strives to find a general law, a concise scientific extract, of things seen, heard, and felt. The abstract nature of things cannot be a tangible object. It is a concept of theory, of science, of the faculty of thought. The understanding of heat consists in singling out that which is common to all phenomena of heat, which is essential or true for all heat. Practically the nature of heat consists of the sum total of all its manifestations, theoretically in its concept, scientifically in the analysis of this concept. To analyze the concept of heat means to ascertain that which is common to all manifestations of heat.

The general nature of the thing is its true nature, the general quality its true quality. We define rain more truly as being wet than as being fertilizing, because it gives moisture wherever it falls, while it fertilizes only under certain circumstances and in certain places. My true friend is one who is constant and loyal to me all my life under all circumstances. Of course, we must not believe in any absolute and unconditional friendship any more than in any absolute and eternal truth. Perfectly true, perfectly universal, is only the general existence, the universe, the absolute quantity. But the real world is absolutely relative, absolutely perishable, an infinity of manifestations, an infinity of qualities. All truths are simply parts of this world, partial truths. Semblance and truth flow dialectically into one another like hard and soft, good and bad, right and wrong, but at the same time they remain different. Even though I know that there is no rain which is "fertile in itself," and no friend who is true in an absolute sense, I may nevertheless refer to a certain rain as fertile in relation to certain crops, and I may distinguish between my more or less true friends.

The universe is the truth. The universe is that which is universal, that is, things which exist and are perceived. The general mark of truth is existence, because universal existence is truth. Now, existence is not a general abstraction, but a reality in the concrete form of sense perceptions. The world of sense perceptions has its true and perceptible existence in the passing and manifold manifestations of nature and life. Therefore all manifestations are recognized as relative truths, all truths as concrete and temporal manifestations. The manifestation of practice is considered as a truth in theory, and vice versa, the truth of theory is manifested in practice. Opposites are mutually relative. Truth and error differ only comparatively, in volume of degree, like being and seeming, life and death, light and dark, like all other opposites in the world. It is a matter of course that all things of this world are worldly, consequently are of the same matter, the same nature, the same family, the same quality. In other words, every volume of perceptible manifestation forms in contact with the human faculty of thought a being, a truth, a general thing. For our consciousness, every particle of dust as well as every dust cloud, or any other mass of material manifestations, is on the one hand an abstract "thing in itself," and on the other a passing phenomenon of the absolute object, the universe. Inside of this universe the various manifestations are systematized or generalized at will and on purpose by means of our mind. The chemical element is as much a manysided system as the organic cell or the whole vegetable kingdom. The smallest and the largest being is divided into individuals, species, families, classes, etc. This systematization, this generalization, this generation of beings is continued in an ascending scale up to the infinity of the universe, and in the descending scale down to the infinity of the parts. In the eyes of the faculty of thought all qualities become abstract things, all things relative qualities.

Every thing, every sense perception, no matter how subjective or shortlived it may be, is true, is a certain part of truth. In other words, the truth exists, not only in the general existence, but every concrete existence has also its own distinct generality or truth. Every object, whether it be a mere passing idea, or a volatile scent, or some tangible matter, constitutes a sum of manifold phenomena. The faculty of thought turns various quantities into one, discerns the equality in different things, seeks the unity in the multiplicity. Mind and matter have at least actual existence in common. Organic nature agrees with inorganic nature in being material. It is true that there are wide divergences between man, monkey, elephant, and plants attached to the soil, but even greater differences are reconciled under the term "organism." However much a stone may differ from a human heart, thinking reason will discover innumerable similarities in them. They at least agree in being matter, they are both visible, tangible, and may be weighed, etc. Their differences are as manifold as their likenesses. Solomon truly says that there is nothing new under the sun, and Schiller also says truly that the world grows old and again grows young. What abstract thing, being, existence, generality is there that is not manifold in its sense manifestations, and individually different from all other things? There are no two drops of water alike. I am now in many respects different from what I was an hour ago, and the likeness between my brother and myself is only relatively greater than the likeness between a watch and an oyster. In short, the faculty of thought is a faculty of absolute generalization, it classes all things without exception under one head, it comprises and understands everything uniformly, while sense perceptions show absolutely everything in a different, new and individual light.

If we apply this metaphysics[1] to our study, the faculty of thought, we see that its functions, like all other things, are material manifestations, which are all equally true. All manifestations of the mind, all ideas, opinions, errors, partake of a certain truth, all of them have a kernel of truth. Just as inevitably as a painter derives all forms of his creation from perceptible objects around him, so are all ideas, images of true things, theories of true objects. So far as perceptions are perceptions, it is a matter of course that all perceptions perceive something. So far as knowledge is knowledge, it requires no explanation that all knowledge knows something. This follows from the rule of identity, according to which a equals a, or from the rule of contradiction, according to which 100 is not 1,000.

All perceptions are thoughts. One might claim, on the other hand, that all thoughts are not perceptions. One might define "perceiving" as a special kind of thought, as real objective thought in distinction from supposing, believing, or imagining. But it cannot be denied that all thoughts have a common nature, in spite of their many differences. Thought is treated in the court of the faculty of thought like all other things, it is made uniform. No matter how different the thoughts I had yesterday may be from those I have to-day, no matter how much the thoughts of different human beings may vary at different times, no matter how clearly we may distinguish between such thoughts as those expressed by the terms idea, conception, judgment, conclusion, impression, etc., they each and all possess the same common and universal nature, because all of them are manifestations of mind.

It follows, then, that the difference between true and erroneous thoughts, between understanding and misunderstanding, like all other differences, is only relative. A thought "in itself" is neither false nor true, it is either of these only in relation to some other object. Thoughts, conceptions, theories, natures, truths, all have this in common that they belong to some object. We have seen that any object is a part of the multiplicity of sense perceptions in the world outside of our brains. After as much of the universal being as constitutes the object which is to be understood has been defined by some customary term of language, truth is to be found in the discovery of the general nature of this perceptible part of being.

The perceptible parts of being which constitute the things of this world have not only a semblance and manifestation, but also a true nature which is given by means of their manifestation. The nature of things is as infinite in number as the world of sense perceptions is infinitely divisible in space and time. Every part of any phenomenon has its own nature, every special phenomenon has its general truth. A phenomenon is perceived in touch with the senses, while the true or essential nature of things is perceived in contact with our faculty of thought. In this way we find ourselves face to face with the necessity of speaking here, where the nature of things is up for discussion, simultaneously of the faculty of thought, and on the other hand of dealing with the nature of things when the faculty of thought is our main subject.

We said at the outset: The criterion of truth includes the criterion of reason. Truth, like reason, consists in developing a general concept, or an abstract theory, from a given sum of sense perceptions. Therefore it is not abstract truth which is the criterion of true understanding, but we rather refer to that understanding as being true which produces the truth, or the general hall-mark of any concrete object. Truth must be objective, that is to say it must be the truth about some concrete object. Perceptions cannot be true to themselves, they are true only in relation to some definite object, and to some outside facts. The work of understanding consists in the abstraction of the general hall-mark from concrete objects. The concrete is the measure of the general, the standard of truth. Whatever is, is true, no matter how much or how little true it may be. Once we have found existence, its general nature follows as truth itself. The difference between that which is more or less general, between being and seeming, between truth and error, is limited to definite conditions, for it presupposes the relation to some special object. Whether a perception is true or false will, therefore, depend not so much on perception as on the scope of the question which perception tries to solve of its own accord or which it is called upon to solve by external circumstances. A perfect understanding is possible only within definite limits. A perfect truth is one which is always aware of its imperfection. For instance, it is perfectly true that all bodies have weight only because the concept of "body" has previously been limited to things which have weight. After reason has assigned the conception of "body in general" to things of various weights, it is no longer a matter for surprise to find that bodies must inevitably have weight. Once it is assumed that the term "bird" was abstracted exclusively from flying animals, we may be sure that all birds fly, whether they are in heaven, on earth, or in any other place. And to explain this we do not require the belief in a priori conceptions which are supposed to differ from empirical conceptions by their strict necessity and generality. Truths are valid only under certain conditions, and under certain conditions errors may be true. It is a true perception that the sun is shining, provided we understand that the sky is not covered by clouds. And it is no less true that a straight stick becomes crooked in flowing water, provided we understand that this truth is an optical one. Truth is that which is common or general to our reasoning faculty within a given circle of sense perceptions. To call within a definite circle of sense perceptions that which is exceptional or special the rule or the general, is error. Error, the opposite of truth, arises when the faculty of thought, or consciousness, inadvertently or shortsightedly and without previous experience concedes to certain phenomena a more general scope than is supported by the senses, for instance when it hastily attributes to what is in fact only an optical existence, a supposed plastic existence also.

The judgment of error is a prejudice. Truth and error, understanding and misunderstanding, knowing and not knowing, have their common habitation in the faculty of thought which is the organ of science. Thought at large is the general expression of experienced facts perceived by the senses, and it includes errors as well. Error is distinguished from truth in that the former assigns to any definite fact of which it is a manifestation, a wider and more general existence than is supported by sense perceptions and experience. Unwarranted assumption is the nature of error. A glass bead does not become a counterfeit, until it pretends to be a genuine pearl.

Schleiden says of the eye: "When the excited blood expands the veins and presses on the nerves, we feel it in the fingers as pain, we see it in the eyes as forked lightning. And thus we obtain the irrefutable proof that our conceptions are free creations of the mind, that we do not perceive the external world as it really is, but that its reflex actions on us simply give rise to a peculiar brain activity, on our part. The products of this activity are frequently connected with certain processes of the external world, but frequently they are not. We close our eyes and we see a circle of light, but there is in reality no shining body. It is easy to see that this may be a great and dangerous source of errors of all kinds. From the teasing forms of a misty moonlight night to the threatening and insanity-producing visions of the believer in ghosts we meet a series of illusions which are not derived from any direct processes of external nature, but belong to the field of the free activity of the mind which is subject to error. It requires great judgment and wide education, before the mind learns to break away from all its own errors and to control them. Reading in general seems so easy, and yet it is a difficult art. It is only by degrees that the mind learns to understand which of the messages of the nerves may be trusted and used as a basis for conceptions. The light, if we consider it entirely by itself, is not clear, not yellow, nor blue nor red. The light is a movement of a very fine and everywhere diffused substance, the ether."

The beautiful world of light and splendor, of color and form, is supposed not to be a perception of something which really is. "Through the thick covering of the grape arbor, a ray of sunlight undulates into the cooling shadows. You think you see the ray of light itself, but what you really see is nothing but a flock of dust particles." The truth about light and color is said to be that they are "waves rushing through ether in restless succession at the rate of 160,000 miles per second." This true physical nature of light and color is supposed to be so illusive, that "it required the sharp intellects of the greatest thinkers to reveal to us this true nature of light. We find that every one of our senses is susceptible only to definite external influences, and that the stimulation of different senses produces different conceptions in our mind. Thus the sense organs are the mediators between the external soulless world (undulations of the ether), which is revealed to us by science, and the beautiful world of sense perceptions in which we find ourselves with our minds."

Schleiden thus gives an illustration of the fact that there is still a great deal of embarrassment, even in our times, when the understanding of these two worlds is under discussion, that there is still much helpless groping to explain the connection between the world of thought, of knowledge or science, which is in this case represented by undulations of the ether, and between the world of our five senses, represented by the bright and colored lights of the eyes or of reality. At the same time this illustration shows how queer the traditional survivals of speculative philosophy sound in the mouth of a modern scientist. The confused condition of this mode of thought is seen in the distinction between "an external sense-perceived world of science" and another one, "in which we find ourselves with our minds." The distinction between the senses and the mind, between theory and practice, between the special and the general, between truth and error, has been noticed by such thinkers, but they have no solution for it. They know there is something missing, but they do not know where to look for it, and therefore they are confused.

The great scientific achievement of the XIXth century consists in the victory over speculation, over knowledge without sense perception, in the delivery of the senses from the thraldom of such knowledge, and in the foundation of empirical investigation. To acknowledge the theoretical value of this achievement means to come to an understanding about the source of error. Contrary to a philosophy that tries to discover truth with the mind, and error with the senses, we seek for truth with the senses and regard the mind as the source of errors. The belief in certain messages of the nerves which are alone worthy of confidence and which can be understood only by degrees without any specific mark of distinction, is a superstition. Let us have confidence in all testimonials of the senses. There is nothing false to be separated from the genuine. The supernatural mind idea is the only deceiver whenever it undertakes to disregard the sense perceptions, and, instead of being the interpreter of the senses, tries to enlarge their statements and repeat what has not been dictated. The eye, in seeing forked lightning or radiant circles when the blood is excited or a pressure exerted on it, perceives no more errors than it does in perceiving any other manifestation of the external world. It is our faculty of thought which makes a mistake, by regarding without further inquiry such subjective events as objective bodies. One who sees ghosts does not commit any mistake, until he claims that his personal apparition is a general phenomenon, until he prematurely takes something for an experience which he has not experienced. Error is an offense against the law of truth which prescribes to our consciousness that it must remember the limits within which a perception is true, or general. Error makes out of something special a generality, out of a predicate a subject, and takes the part for the whole. Error makes a priori conclusions, while truth, its opposite, arrives at understanding by a posteriori reasoning.

A priori and a posteriori understanding are related in the same way as philosophy and natural science, taking the latter in the widest meaning of the term, that of science in general. The contrast between believing and knowing is duplicated in that between philosophy and natural science. Speculative philosophy, like religion, lives on faith. The modern world has transformed faith into science. The reactionists in politics who demand that science retrace its steps desire its return to faith. The content of faith is acquired without exertion. Faith makes a priori perceptions, while science arrives at its knowledge by hard a posteriori study. To give up faith means to give up taking things easy. And to confine science to a posteriori knowledge means to decorate it with the characteristic mark of modern times, work.

It is not a result of scientific study, but merely a freak of philosophy on the part of Schleiden to deny the reality and truth of light phenomena, to call them fantasmagoria created by the free play of the mind. His superstitious belief in philosophical speculation misleads him into abandoning the scientific method of induction and speaking of "waves rushing through ether in restless succession at the rate of 160,000 miles per hour" as being the real and true nature of light and color, in contradistinction to the color phenomena of light. The perversion of this mode of procedure becomes evident by his referring to the material world of the eyes as a "creation of the mind" and to the undulations of the ether, revealed by the "sharp intellect of the greatest thinkers" as "physical nature."

The truth of science maintains the same relation to the sense perception that the general does to the special. Waves of light, the so-called truth of light and color, represent the "true" nature of light only in so far as they represent what is common to all light phenomena, whether they are white, yellow, blue, or any other color. The world of the mind, or of science finds its raw material, its premise, its proof, its beginning, and its boundary in sense perception.

When we have learned that the nature, or the truth, of things is not back of their phenomena, but can be perceived only by the help of phenomena, and that it does not exist "in itself," but only in connection with the faculty of understanding, that the nature is separated from the phenomena only by thought; and when we see on the other hand, that the faculty of understanding does not derive conceptions out of itself, but only out of their relations with some phenomenon; then this discussion of the "nature of things" is an evidence that the nature of the faculty of thought is a conception which we have obtained from its sense manifestations. To understand that the faculty of thought, although universal in the choice of its objects, is nevertheless limited in that it requires some object; to recognize that the true thought process, that is to say the thought with a scientific result, differs from unscientific thinking by consciously attaching itself to some external object; to realize that truth, or universality, is not perceived "in itself," but can be perceived only by means of some given object; this frequently varied statement reveals the nature of the faculty of thought. This statement re-appears at the end of every chapter, because all special truths, all special chapters, serve only to demonstrate the general chapter of universal truth.