In the last chapter we discussed whether the world has a material or spiritual nature. We came to the conclusion that all phenomena in the world are phenomena of moving matter or material movement. We further said that even thought is a material quality, a quality which is bound up with a special organ, the brain. It can be said of this matter that it is as infinitely manifold as it is absolutely unitary. Chemists and physicists are approaching ever nearer to the unity of matter through the separation of matter into atoms and of atoms into smaller, homogeneous particles. On the other hand we see how this unitary matter is infinitely varied to form various substances. And nature is not the only source of an unlimited number of different substances; there is also man, who has added still more substances to those of nature. This happens, as you know, in the chemistry laboratory, where matter is created which is not present in nature. What is true of matter is also true of motion, which is inseparably linked with matter. Motion too is absolutely unitary as well as absolutely manifold, infinitely multiform. From the simplest locomotion up to thought runs an infinitely manifold chain of material forms of activity.
We come now to the next fundamental problem, to the problem of the relation of thought to reality. These are the questions: are things knowable as they are in themselves? Is the essence of things knowable or are only phenomena knowable? Otherwise expressed: is truth knowable? Further: is this truth knowable as a whole or only in part? Can thought know things without limit, or are there bounds, limits of knowledge which are resident in the nature of thought itself?
And the next question, which is connected with the above: has truth certain distinguishing marks and what are they?
I will first give the objections which the idealistic world-view makes against our being able to know things as they are in reality, or our being able to know the essence of things. The idealistic world-view says: it is not possible to know the essence of things in themselves because all knowledge comes about only through the medium of thought. Through thought, however, things are not absorbed as they are in themselves, but they become changed. Thought is a tool, and like every tool thought alters the matter to which it is applied. Just as the potter changes the clay which he works by giving it a certain form, so thought recasts the things which it wants to know. To this one might reply: we will know things as they are in themselves if we remove the form which thought gives to them. But if we remove this form, then they remain outside of thought. Hence, the dilemma, the contradiction, appears thus: either things remain outside of thought, in which case they are not accessible to knowledge, or they enter into thought, in which case they become transformed by thought so that we can in no instance know how they exist in reality. This is the position of the idealistic world-view.
To this we answer: what you idealists demand is something meaningless, something which contradicts the nature of things. Since thought is associated with things, what occurs is no different from what occurs in any association of two things: when two things become associated they mutually affect each other. Thing A acts on thing B, and B on A. The sun attracts the earth and the earth attracts the sun. The sun influences the earth and the earth influences the sun. No action without reaction. Through action and reaction the nature of both things becomes manifest. To reject the effect of a thing upon other things is to reject the thing itself. Things act on thought and thought acts on things. The relationship of thought is no different from the interaction between two things in general. It is absurd to ask that knowledge arise without thought working upon things, that is, to ask that a reaction occur without the occurrence of an action. With the rejection of the reaction the action is rejected, and with both together the thing itself or the essence of the thing is rejected. This is a contradiction, and not dialectical, but metaphysical. It is just as if it were expected that the stomach digest food without eating it and without acting on it.
Idealism further says: man cannot know the essence of things as they are in themselves because his sense organs are of a very special kind and apprehend things in a very special way. We know that certain color vibrations are apprehended by human eyes as blue, but to a bee or an ant this color may appear not as blue but somewhat like grey, or perhaps some other color. Human sense organs perceive things in their own peculiar way, a way which is different from the perception of these things by other organisms.
Or let us take another example, an example from the sense of smell. We know that there are certain plants which have certain smells which attract particular kinds of insects. There are plants which have a smell like decayed meat, like carrion. This smell repels men, but certain animals are drawn to these plants by such smells. It can therefore be assumed that this smell affects these animals differently from man. These examples can be piled up; we could go on, for instance, to the feeling capacity. It can certainly be assumed that a certain temperature which to men feels cold, is otherwise felt by a cold-blooded animal, by a fish, for example. Similarly in the realm of tones. Certainly the tone-sensation of the insect or the fish is different from that of man. I cite all these things in order to point out that human sense organs (eyes, ears, etc.) have a peculiar character and are different from those of other organisms in their mode of apprehending things. From this follows the objection of idealists that human knowledge is not an apprehension of things as they are, but a peculiar transformation which conforms not only to the nature of human thought, but also to human sense organs.
The second point is that not only are the sense organs of man of a peculiar type, different from those of other organisms, but also that these sense organs are limited in their perception. And one might raise the question: are there not things, phenomena, which are not at all accessible to human senses; For example: we know that there are colors which cannot be perceived by human eyes, but whose presence can be established by other means. These are the colors at the extremes of the so-called color spectrum, ultra-violet and infra-red. This applies not only to the perception of colors and differences in colors, but also to the perception of differences in brightness. Night-animals like the owl or the cat observe differences in the degree of brightness at night which cannot be observed by human eyes. Similarly for the other realms of sense perception. The range of perception of every sense organ has upper and lower limits, quantitative and qualitative, just as within its range it has quantitative and qualitative limits of discrimination.
To this we answer that man has a peculiar means of overcoming the limitation and peculiarity of his sense instruments. This means is thought. It may be that the dog has a sharper nose, the eagle a sharper eye than man, and that other animals can perceive other things better than man. Nevertheless the intellectual capacity of man extends far beyond that of any other organism because he is able to elevate himself, through thought, above the peculiarity and limitations of his senses, and not only through thought, but also through his hand directed by thought, through the construction of artificial organs, through tools. I need not enumerate the telescope, the microscope, the instruments and apparatus by means of which man artificially extends his sense organs, makes them keener and more exact. The main point here is that human thought overcomes the peculiarities of human sense organs.
For example: colors, as the physicist conceives them, arc reduced to vibrations of a certain material medium, to something which no longer has direct relation to human vision. Or in the case of tones and noises, the physicist reduces them to air vibrations. This too is no longer bound up with direct perception through the car. Thus science, thought, knows how to overcome the peculiarities of human sense perceptions. But one may ask the question: what about the limitation of human senses? Is it not possible that there are certain qualities of things which are not perceivable through human senses? I have already said that there are certain colors which man cannot see with the naked eye, ultra-violet and infra-red. But how does one know of these colors, how does one perceive them?
Through special physical instruments. In the last analysis all properties of things are accessible to man, if not directly, then indirectly, if not with the naked eye, then through artificial organs. This is the case because there are no properties in things which do not have some effect or other, and because the effects form a connected chain which can follow from one link to another. Still another example: above a certain temperature I cannot perceive heat with my bare hand, with my skin, but as a physicist or a technician I measure this heat with a specially constructed thermometer. And how do I perceive this thermometer? I read off the degrees by eye, so that in the last analysis I do not perceive the heat with my skin, but with my eve. It must be added that this unlimited perceptibility of things is realized only through an unlimited process - the continuous extension of previous limits. Seen as a whole this extension of limits is a continuous process; viewed closely it is a series of large and small steps.
The further question which we raise is the question of the characteristics of knowledge, the question as to the criterion by which I determine that a proposition which I set up is true. The usual answer is this: I accept it as true when it is not contradictory. Contradiction is the criterion of error. What could be clearer, simpler, and more certain? This self-styled criterion of truth collapses as soon as it is seriously tested. I shall give a few examples: you know that we ascribe three dimensions to space: breadth, length, and height. But the physicist adds time to the three space-dimensions to form four cosmic dimensions. Moreover, if I imagine that the world has ten dimensions, no inner contradiction is involved. Nevertheless, I would not accept such a proposition as a statement of the physical structure of the world.
As you know, there are legends about the sea-serpent. It is said to be a serpent-like animal that swims in the sea and is a hundred to a thousand yards long. In the conception of the sea-serpent there is no contradiction. There is certainly no contradiction in the concept of serpent if I suppose its length to be merely ten to a hundred yards. Or let us take a figure from folk-lore or religion, say the idea of dragons or ghosts. All these ideas are not in themselves contradictions. I can logically conceive of them. The criterion of their falsehood lies not in inner contradictions, but in something else. There is also the reverse situation: I have already pointed out that even in a science like mathematics contradictions occur without necessarily being the kind of inner contradiction which is the criterion of true and false, that is, there can be contradictions without falsity.
I find the actual characteristics of truth not when I compare concepts with each other, but when I compare the concept with reality. This can only be done by observation. It may well be that the idea of ghosts contains no contradiction in itself, but it contains a contradiction when compared to the common experience that mental functions are bound up with bodies. Or take the dragon. I can well conceive of such an animal, but there is none, none exists in reality. This is just another example of fantasy.
The laws according to which the planets move were first set forth by the astronomer, Kepler. I test their correctness and the degree of their exactitude by observing the course of the planets. One of the best ways of determining whether I actually know things is experiment research. If I wish to know whether I have truly discerned that water consists of two elements, of oxygen and hydrogen, which are combined in certain proportions of weight, how do I determine that my contention is correct? Through experiment - of two kinds: first, by bringing oxygen and hydrogen together under certain conditions of temperature and of pressure, and thus producing water; second, by reducing water, through chemical means, into hydrogen and oxygen. Through this experiment I discover that this idea is no delusion but corresponds to the actual nature of the thing. Such experiments are made on a small scale in the chemical laboratory; they arc made on a large scale in industry. Industrial practice is likewise a test of the truth of my perception. Such experiments are not only appropriate in nature, but also in society. Politics in the last analysis is nothing but a series of experiments in the realm of society. If, for example, I set up the law that the small farmers must be won for the revolution in order to partition the land of the great landed proprietors among them, this can be false or true. I learn whether it is true by putting the matter to a test.
We now conclude: practice, the activity of man, is the test of the possibility and extent of his knowing things. If from oxygen and hydrogen I can compose water, then to this extent I have correct knowledge of the nature of water.
The further question arises, is complete or absolute knowledge of things possible? The answer is: I cannot knowany single thing fully and conclusively all at once. The process of knowing a single thing as well as knowing the whole world is endless; that is, complete knowledge of things is realized only through a series of relative and incomplete bits of knowledge. But this series represents absolute or complete knowledge. Accordingly, you arrive at a measure for the relation of the concepts, true and false. In everyday life these opposites are placed absolutely and sharply against each other. A statement is either false or true. There is no third alternative. Knowledge which comes progressively closer to things always contains a bit of truth, but also a bit of untruth, of falsity. I will give a well-known example: the law of gravity, the universal law which governs the motion of the planets around the sun, was first recognized by the great English natural scientist, Newton, in the seventeenth century. Until the twentieth century this was considered satisfactory and correct - until Einstein presented a more exact formulation of this law. But it would be childish simply to say that Newton's law is now false, that Einstein's law is true. Newton's law contains an extraordinarily great approximation to the truth and at the same time an element of inexactitude. The Einstein law contains a greater element of truth and a lesser element of untruth or inexactitude.The true and the false are contained in both. But the last, the Einsteinian, is a greater approximation to absolute truth than the Newtonian law.
Closely connected with this is the question whether I can fully know the world as a whole, as distinguished from the question whether I can fully know a single part of the world. Can I fully know the world as a whole? To this we say: yes, it is possible. But one cannot swallow the world as a whole all at once; it is too big. One knows the whole of the world through its individual parts. One must penetrate into the special sciences in order to know the world. With the progress of science the picture becomes ever more manifold. Inversely, I can say that the universal idea of the world as a whole is the premise of every individual science. Without the premise that all things form a unitary world, I have no starting point for the individual science. The relation is such that the individual sciences like physics and botany presuppose the science of the world as a whole, and inversely this science of the world as a whole is realized only through the individual sciences. The universal world-concept is a matter of dialectics, and accordingly we can say: the individual sciences presuppose dialectics, and dialectics presupposes the individual sciences. Each conditions the other.
Finally, the last question which I will briefly answer is a question which agitated thinkers for many centuries: are there innate ideas in the human mind? Has man certain concepts in his head with which he comes into the world and which he does not first have to learn through experience? The answer is: there are no innate particular concepts of the cat or the dog or the donkey or the tree or the camel. Man learns everything through experience. There also are no innate general concepts but there is an "innate" fundamental quality of thought, a natural fundamental characteristic of thought, just as salt, water, and ice have their characteristics. Of this fundamental quality or fundamental function of thought we will speak in greater detail in the next chapter. But I wish to add this: this basic function of thought operates only in relation to sensory experience. Moreover, thought behaves just like the other organs, for example, the stomach. The stomach digests only when it is given something to digest. The basic function of thought manifests itself only when matter is at hand for it.