But to institute an inquiry according to this criterion, as to whether something was dreamt or seen, would always be difficult and often impossible. For we are by no means in a position to trace link by link the causal connection between any experienced event and the present moment, but we do not on that account explain it as dreamt. Therefore in real life we do not commonly employ that method of distinguishing between dreams and reality. The only sure criterion by which to distinguish them is in fact the entirely empirical one of awaking, through which at any rate the causal connection between dreamed events and those of waking life, is distinctly and sensibly broken off. This is strongly supported by the remark of Hobbes in the second chapter of Leviathan, that we easily mistake dreams for reality if we have unintentionally fallen asleep without taking off our clothes, and much more so when it also happens that some undertaking or design fills all our thoughts, and occupies our dreams as well as our waking moments. We then observe the awaking just as little as the falling asleep, dream and reality run together and become confounded. In such a case there is nothing for it but the application of Kant's criterion; but if, as often happens, we fail to establish by means of this criterion, either the existence of causal connection with the present, or the absence of such connection, then it must for ever remain uncertain whether an event was dreamt or really happened. Here, in fact, the intimate relationship between life and dreams is brought out very clearly, and we need not be ashamed to confess it, as it has been recognised and spoken of by many great men. The Vedas and Puranas have no better simile than a dream for the whole knowledge of the actual world, which they call the web of Mâyâ, and they use none more frequently. Plato often says that men live only in a dream; the philosopher alone strives to awake himself. Pindar says (ii. η. 135): σκιας οναρ ανθρωπος (umbræ somnium homo), and Sophocles: —

 
Ὀνω γυν ἡμας ουδεν οντας αλλο, πλην
Σιδωλ᾽ ὁσοιπερ ζωμεν, ὴ κουφην σκιαν. – Ajax, 125.
 

(Nos enim, quicunque vivimus, nihil aliud esse comperio quam simulacra et levem umbram.) Beside which most worthily stands Shakespeare: —

 
“We are such stuff
As dreams are made on, and our little life
Is rounded with a sleep.” —Tempest, Act iv. Sc. 1.
 

Lastly, Calderon was so deeply impressed with this view of life that he sought to embody it in a kind of metaphysical drama – “Life a Dream.”

After these numerous quotations from the poets, perhaps I also may be allowed to express myself by a metaphor. Life and dreams are leaves of the same book. The systematic reading of this book is real life, but when the reading hours (that is, the day) are over, we often continue idly to turn over the leaves, and read a page here and there without method or connection: often one we have read before, sometimes one that is new to us, but always in the same book. Such an isolated page is indeed out of connection with the systematic study of the book, but it does not seem so very different when we remember that the whole continuous perusal begins and ends just as abruptly, and may therefore be regarded as merely a larger single page.

Thus although individual dreams are distinguished from real life by the fact that they do not fit into that continuity which runs through the whole of experience, and the act of awaking brings this into consciousness, yet that very continuity of experience belongs to real life as its form, and the dream on its part can point to a similar continuity in itself. If, therefore, we consider the question from a point of view external to both, there is no distinct difference in their nature, and we are forced to concede to the poets that life is a long dream.

Let us turn back now from this quite independent empirical origin of the question of the reality of the outer world, to its speculative origin. We found that this consisted, first, in the false application of the principle of sufficient reason to the relation of subject and object; and secondly, in the confusion of its forms, inasmuch as the principle of sufficient reason of knowing was extended to a province in which the principle of sufficient reason of being is valid. But the question could hardly have occupied philosophers so constantly if it were entirely devoid of all real content, and if some true thought and meaning did not lie at its heart as its real source. Accordingly, we must assume that when the element of truth that lies at the bottom of the question first came into reflection and sought its expression, it became involved in these confused and meaningless forms and problems. This at least is my opinion, and I think that the true expression of that inmost meaning of the question, which it failed to find, is this: – What is this world of perception besides being my idea? Is that of which I am conscious only as idea, exactly like my own body, of which I am doubly conscious, in one aspect as idea, in another aspect as will? The fuller explanation of this question and its answer in the affirmative, will form the content of the second book, and its consequences will occupy the remaining portion of this work.

§ 6. For the present, however, in this first book we consider everything merely as idea, as object for the subject. And our own body, which is the starting-point for each of us in our perception of the world, we consider, like all other real objects, from the side of its knowableness, and in this regard it is simply an idea. Now the consciousness of every one is in general opposed to the explanation of objects as mere ideas, and more especially to the explanation of our bodies as such; for the thing in itself is known to each of us immediately in so far as it appears as our own body; but in so far as it objectifies itself in the other objects of perception, it is known only indirectly. But this abstraction, this one-sided treatment, this forcible separation of what is essentially and necessarily united, is only adopted to meet the demands of our argument; and therefore the disinclination to it must, in the meantime, be suppressed and silenced by the expectation that the subsequent treatment will correct the one-sidedness of the present one, and complete our knowledge of the nature of the world.

At present therefore the body is for us immediate object; that is to say, that idea which forms the starting-point of the subject's knowledge; because the body, with its immediately known changes, precedes the application of the law of causality, and thus supplies it with its first data. The whole nature of matter consists, as we have seen, in its causal action. But cause and effect exist only for the understanding, which is nothing but their subjective correlative. The understanding, however, could never come into operation if there were not something else from which it starts. This is simple sensation – the immediate consciousness of the changes of the body, by virtue of which it is immediate object. Thus the possibility of knowing the world of perception depends upon two conditions; the first, objectively expressed, is the power of material things to act upon each other, to produce changes in each other, without which common quality of all bodies no perception would be possible, even by means of the sensibility of the animal body. And if we wish to express this condition subjectively we say: The understanding first makes perception possible; for the law of causality, the possibility of effect and cause, springs only from the understanding, and is valid only for it, and therefore the world of perception exists only through and for it. The second condition is the sensibility of animal bodies, or the quality of being immediate objects of the subject which certain bodies possess. The mere modification which the organs of sense sustain from without through their specific affections, may here be called ideas, so far as these affections produce neither pain nor pleasure, that is, have no immediate significance for the will, and are yet perceived, exist therefore only for knowledge. Thus far, then, I say that the body is immediately known, is immediate object. But the conception of object is not to be taken here in its fullest sense, for through this immediate knowledge of the body, which precedes the operation of the understanding, and is mere sensation, our own body does not exist specifically as object, but first the material things which affect it: for all knowledge of an object proper, of an idea perceived in space, exists only through and for the understanding; therefore not before, but only subsequently to its operation. Therefore the body as object proper, that is, as an idea perceived in space, is first known indirectly, like all other objects, through the application of the law of causality to the action of one of its parts upon another, as, for example, when the eye sees the body or the hand touches it. Consequently the form of our body does not become known to us through mere feeling, but only through knowledge, only in idea; that is to say, only in the brain does our own body first come to appear as extended, articulate, organic. A man born blind receives this idea only little by little from the data afforded by touch. A blind man without hands could never come to know his own form; or at the most could infer and construct it little by little from the effects of other bodies upon him. If, then, we call the body an immediate object, we are to be understood with these reservations.

In other respects, then, according to what has been said, all animal bodies are immediate objects; that is, starting-points for the subject which always knows and therefore is never known in its perception of the world. Thus the distinctive characteristic of animal life is knowledge, with movement following on motives, which are determined by knowledge, just as movement following on stimuli is the distinctive characteristic of plant-life. Unorganised matter, however, has no movement except such as is produced by causes properly so called, using the term in its narrowest sense. All this I have thoroughly discussed in my essay on the principle of sufficient reason, § 20, in the “Ethics,” first essay, iii., and in my work on Sight and Colour, § 1, to which I therefore refer.

It follows from what has been said, that all animals, even the least developed, have understanding; for they all know objects, and this knowledge determines their movements as motive. Understanding is the same in all animals and in all men; it has everywhere the same simple form; knowledge of causality, transition from effect to cause, and from cause to effect, nothing more; but the degree of its acuteness, and the extension of the sphere of its knowledge varies enormously, with innumerable gradations from the lowest form, which is only conscious of the causal connection between the immediate object and objects affecting it – that is to say, perceives a cause as an object in space by passing to it from the affection which the body feels, to the higher grades of knowledge of the causal connection among objects known indirectly, which extends to the understanding of the most complicated system of cause and effect in nature. For even this high degree of knowledge is still the work of the understanding, not of the reason. The abstract concepts of the reason can only serve to take up the objective connections which are immediately known by the understanding, to make them permanent for thought, and to relate them to each other; but reason never gives us immediate knowledge. Every force and law of nature, every example of such forces and laws, must first be immediately known by the understanding, must be apprehended through perception before it can pass into abstract consciousness for reason. Hooke's discovery of the law of gravitation, and the reference of so many important phenomena to this one law, was the work of immediate apprehension by the understanding; and such also was the proof of Newton's calculations, and Lavoisier's discovery of acids and their important function in nature, and also Goethe's discovery of the origin of physical colours. All these discoveries are nothing more than a correct immediate passage from the effect to the cause, which is at once followed by the recognition of the ideality of the force of nature which expresses itself in all causes of the same kind; and this complete insight is just an example of that single function of the understanding, by which an animal perceives as an object in space the cause which affects its body, and differs from such a perception only in degree. Every one of these great discoveries is therefore, just like perception, an operation of the understanding, an immediate intuition, and as such the work of an instant, an apperçu, a flash of insight. They are not the result of a process of abstract reasoning, which only serves to make the immediate knowledge of the understanding permanent for thought by bringing it under abstract concepts, i. e., it makes knowledge distinct, it puts us in a position to impart it and explain it to others. The keenness of the understanding in apprehending the causal relations of objects which are known indirectly, does not find its only application in the sphere of natural science (though all the discoveries in that sphere are due to it), but it also appears in practical life. It is then called good sense or prudence, as in its other application it is better called acuteness, penetration, sagacity. More exactly, good sense or prudence signifies exclusively understanding at the command of the will. But the limits of these conceptions must not be too sharply defined, for it is always that one function of the understanding by means of which all animals perceive objects in space, which, in its keenest form, appears now in the phenomena of nature, correctly inferring the unknown causes from the given effects, and providing the material from which the reason frames general rules as laws of nature; now inventing complicated and ingenious machines by adapting known causes to desired effects; now in the sphere of motives, seeing through and frustrating intrigues and machinations, or fitly disposing the motives and the men who are susceptible to them, setting them in motion, as machines are moved by levers and wheels, and directing them at will to the accomplishment of its ends. Deficiency of understanding is called stupidity. It is just dulness in applying the law of causality, incapacity for the immediate apprehension of the concatenations of causes and effects, motives and actions. A stupid person has no insight into the connection of natural phenomena, either when they follow their own course, or when they are intentionally combined, i. e., are applied to machinery. Such a man readily believes in magic and miracles. A stupid man does not observe that persons, who apparently act independently of each other, are really in collusion; he is therefore easily mystified, and outwitted; he does not discern the hidden motives of proffered advice or expressions of opinion, &c. But it is always just one thing that he lacks – keenness, rapidity, ease in applying the law of causality, i. e., power of understanding. The greatest, and, in this reference, the most instructive example of stupidity I ever met with, was the case of a totally imbecile boy of about eleven years of age, in an asylum. He had reason, because he spoke and comprehended, but in respect of understanding he was inferior to many of the lower animals. Whenever I visited him he noticed an eye-glass which I wore round my neck, and in which the window of the room and the tops of the trees beyond were reflected: on every occasion he was greatly surprised and delighted with this, and was never tired of looking at it with astonishment, because he did not understand the immediate causation of reflection.

While the difference in degree of the acuteness of the understanding, is very great between man and man, it is even greater between one species of animal and another. In all species of animals, even those which are nearest to plants, there is at least as much understanding as suffices for the inference from the effect on the immediate object, to the indirectly known object as its cause, i. e., sufficient for perception, for the apprehension of an object. For it is this that constitutes them animals, as it gives them the power of movement following on motives, and thereby the power of seeking for food, or at least of seizing it; whereas plants have only movement following on stimuli, whose direct influence they must await, or else decay, for they cannot seek after them nor appropriate them. We marvel at the great sagacity of the most developed species of animals, such as the dog, the elephant, the monkey or the fox, whose cleverness has been so admirably sketched by Buffon. From these most sagacious animals, we can pretty accurately determine how far understanding can go without reason, i. e., abstract knowledge embodied in concepts. We could not find this out from ourselves, for in us understanding and reason always reciprocally support each other. We find that the manifestation of understanding in animals is sometimes above our expectation, and sometimes below it. On the one hand, we are surprised at the sagacity of the elephant, who, after crossing many bridges during his journey in Europe, once refused to go upon one, because he thought it was not strong enough to bear his weight, though he saw the rest of the party, consisting of men and horses, go upon it as usual. On the other hand, we wonder that the intelligent Orang-outangs, who warm themselves at a fire they have found, do not keep it alight by throwing wood on it; a proof that this requires a deliberation which is not possible without abstract concepts. It is clear that the knowledge of cause and effect, as the universal form of understanding, belongs to all animals a priori, because to them as to us it is the prior condition of all perception of the outer world. If any one desires additional proof of this, let him observe, for example, how a young dog is afraid to jump down from a table, however much he may wish to do so, because he foresees the effect of the weight of his body, though he has not been taught this by experience. In judging of the understanding of animals, we must guard against ascribing to it the manifestations of instinct, a faculty which is quite distinct both from understanding and reason, but the action of which is often very analogous to the combined action of the two. We cannot, however, discuss this here; it will find its proper place in the second book, when we consider the harmony or so-called teleology of nature: and the 27th chapter of the supplementary volume is expressly devoted to it.

Deficiency of understanding we call stupidity: deficiency in the application of reason to practice we shall recognise later as foolishness: deficiency of judgment as silliness, and lastly, partial or entire deficiency of memory as madness. But each of these will be considered in its own place. That which is correctly known by reason is truth, that is, an abstract judgment on sufficient grounds (Essay on the Principle of Sufficient Reason, § 29 and following paragraphs); that which is correctly known by understanding is reality, that is correct inference from effect on the immediate object to its cause. Error is opposed to truth, as deception of the reason: illusion is opposed to reality, as deception of the understanding