kant’s mistaken formulation of the problem has had a greater or lesser influence on all subsequent students of the theory of knowledge. for kant, the view that all objects which are given to us in experience are ideas in our minds is a consequence of his theory of the a priori. for nearly all his successors, it has become the first principle and starting-point of their epistemological systems. it is said that the first and most immediate truth is, simply and solely, the proposition that we know our own ideas. this has come to be a well-nigh universal conviction among philosophers. g. e. schulze maintains in his ?nesidemus, as early as 1792, that all our cognitions are mere ideas and that we can never transcend our ideas. schopenhauer puts forward, with all the philosophical pathos which distinguishes him, the view that the permanent achievement of kant’s philosophy is the thesis that “the world is my idea.” to eduard von hartmann this thesis is so incontestable, that he addresses his treatise, kritische grundlegung des transcendentalen [292]realismus, exclusively to readers who have achieved critical emancipation from the na?ve identification of the world of perception with the thing-in-itself. he demands of them that they shall have made clear to themselves the absolute heterogeneity of the object of perception which through the act of representation has been given as a subjective and ideal content of consciousness, and of the thing-in-itself which is independent of the act of representation and of the form of consciousness and which exists in its own right. his readers are required to be thoroughly convinced that the whole of what is immediately given to us consists of ideas.1 in his latest work on theory of knowledge, hartmann does, indeed, attempt to give reasons for this view. what value should be attached to these reasons by an unprejudiced theory of knowledge will appear in the further course of our discussions. otto liebmann posits as the sacrosanct first principle of the theory of knowledge the proposition, “consciousness cannot transcend itself.”2 volkelt has called the proposition that the first and most immediate truth is the limitation of all our knowledge, in the first instance, to our own ideas exclusively, the positivistic principle of knowledge. he regards only those theories of knowledge as “in the fullest sense critical” which “place this principle, as the only fixed starting-point of philosophy, at the [293]head of their discussions and then consistently think out its consequences.”3 other philosophers place other propositions at the head of the theory of knowledge, e.g., the proposition that its real problem concerns the relation between thought and being, and the possibility of a mediation between them;4 or that it concerns the way in which being becomes an object of consciousness;5 and many others. kirchmann starts from two epistemological axioms, “whatever is perceived is,” and, “whatever is self-contradictory, is not.”6 according to e. l. fischer, knowledge is the science of something actual, something real,7 and he criticises this dogma as little as does goering who asserts similarly, “to know means always to know something which is. this is a fact which cannot be denied either by scepticism or by kant’s critical philosophy.”8 these two latter thinkers simply lay down the law: this is what knowledge is. they do not trouble to ask themselves with what right they do it.

but, even if these various propositions were correct, or led to correct formulations of the problem, it would still be impossible to discuss them at the outset of the theory of knowledge. for, they all belong, as positive and definite cognitions, within the realm of knowledge. to [294]say that my knowledge extends, in the first instance, only to my ideas, is to express in a perfectly definite judgment something which i know. in this judgment i qualify the world which is given to me by the predicate “existing in the form of idea.” but how am i to know, prior to all knowledge, that the objects given to me are ideas?

the best way to convince ourselves of the truth of the assertion that this proposition has no right to be put at the head of the theory of knowledge, is to retrace the way which the human mind must follow in order to reach this proposition, which has become almost an integral part of the whole modern scientific consciousness. the considerations which have led to it are systematically summarised, with approximate exhaustiveness, in part i of eduard von hartmann’s treatise, das grundproblem der erkenntnistheorie. his statement, there, may serve as a sort of guiding-thread for us in our task of reviewing the reasons which may lead to the acceptance of this proposition.

these reasons are physical, psycho-physical, physiological, and properly philosophical.

the physicist is led by observation of the phenomena which occur in our environment when, e.g., we experience a sensation of sound, to the view that there is nothing in these phenomena which in the very least resembles what we perceive immediately as sound. outside, in the space which surrounds us, nothing is to be found except longitudinal oscillations [295]of bodies and of the air. thence it is inferred that what in ordinary life we call “sound” or “tone” is nothing but the subjective reaction of our organism to these wave-like oscillations. similarly, it is inferred that light and colour and heat are purely subjective. the phenomena of colour-dispersion, of refraction, of interference, of polarisation, teach us that to the just-mentioned sensations there correspond in the outer space certain transverse oscillations which we feel compelled to ascribe, in part to the bodies, in part to an immeasurably fine, elastic fluid, the “ether.” further, the physicist is driven by certain phenomena in the world of bodies to abandon the belief in the continuity of objects in space, and to analyse them into systems of exceedingly minute particles (molecules, atoms), the size of which, relatively to the distances between them, is immeasurably small. thence it is inferred that all action of bodies on each other is across the empty intervening space, and is thus a genuine actio in distans. the physicist believes himself justified in holding that the action of bodies on our senses of touch and temperature does not take place through direct contact, because there must always remain a definite, if small, distance between the body and the spot on the skin which it is said to “touch.” thence it is said to follow that what we sense as hardness or heat in bodies is nothing but the reactions of the end-organs of our touch- and temperature-nerves to the [296]molecular forces of bodies which act upon them across empty space.

these considerations from the sphere of physics are supplemented by the psycho-physicists with their doctrine of specific sense-energies. j. müller has shown that every sense can be affected only in its own characteristic way as determined by its organisation, and that its reaction is always of the same kind whatever may be the external stimulus. if the optical nerve is stimulated, light-sensations are experienced by us regardless of whether the stimulus was pressure, or an electric current, or light. on the other hand, the same external phenomena produce quite different sensations according as they are perceived by different senses. from these facts the inference has been drawn that there occurs only one sort of phenomenon in the external world, viz., motions, and that the variety of qualities of the world we perceive is essentially a reaction of our senses to these motions. according to this view, we do not perceive the external world as such, but only the subjective sensations which it evokes in us.

physiology adds its quota to the physical arguments. physics deals with the phenomena which occur outside our organisms and which correspond to our percepts. physiology seeks to investigate the processes which go on in man’s own body when a certain sensation is evoked in him. it teaches us that the epidermis is wholly insensitive to the stimuli [297]in the external world. thus, e.g., if external stimuli are to affect the end-organs of our touch-nerves on the surface of our bodies, the oscillations which occur outside our bodies have to be transmitted through the epidermis. in the case of the senses of hearing and of sight, the external motions have, in addition, to be modified by a number of structures in the sense-organs, before they reach the nerves. the nerves have to conduct the effects produced in the end-organs up to the central organ, and only then can take place the process by means of which purely mechanical changes in the brain produce sensations. it is clear that the stimulus which acts upon the sense-organs is so completely changed by the transformations which it undergoes, that every trace of resemblance between the initial impression on the sense-organs and the final sensation in consciousness must be obliterated. hartmann sums up the outcome of these considerations in these words: “this content of consciousness consists, originally, of sensations which are the reflex responses of the soul to the molecular motions in the highest cortical centres, but which have not the faintest resemblance to the molecular motions by which they are elicited.”

if we think this line of argument through to the end, we must agree that, assuming it to be correct, there survives in the content of our consciousness not the least element of what may be called “external existence.” [298]

to the physical and physiological objections against so-called “na?ve realism” hartmann adds some further objections which he describes as philosophical in the strict sense. a logical examination of the physical and physiological objections reveals that, after all, the desired conclusion can be reached only if we start from the existence and nexus of external objects, just as these are assumed by the ordinary na?ve consciousness, and then inquire how this external world can enter the consciousness of beings with organisms such as ours. we have seen that every trace of such an external world is lost on the way from the impression on the sense-organ to the appearance of the sensation in our consciousness, and that in the latter nothing survives except our ideas. hence, we have to assume that the picture of the external world which we actually have, has been built up by the soul on the basis of the sensations given to it. first, the soul constructs out of the data of the senses of touch and sight a picture of the world in space, and then the sensations of the other senses are fitted into this space-system. when we are compelled to think of a certain complex of sensations as belonging together, we are led to the concept of substance and regard substance as the bearer of sense-qualities. when we observe that some sense-qualities disappear from a substance and that others appear in their place, we ascribe this event in the world of phenomena to a [299]change regulated by the law of causality. thus, according to this view, our whole world-picture is composed of subjective sensations which are ordered by the activity of our own souls. hartmann says, “what the subject perceives is always only modifications of its own psychic states and nothing else.”9

now let us ask ourselves, how do we come by such a view? the bare skeleton of the line of thought which leads to it is as follows. supposing an external world exists, we do not perceive it as such but transform it through our organisation into a world of ideas. this is a supposition which, when consistently thought out, destroys itself. but is this reflection capable of supporting any positive alternative? are we justified in regarding the world, which is given to us, as the subjective content of ideas because the assumptions of the na?ve consciousness, logically followed out, lead to this conclusion? our purpose is, rather, to exhibit these assumptions themselves as untenable. yet, so far we should have found only that it is possible for a premise to be false and yet for the conclusion drawn from it to be true. granted that this may happen, yet we can never regard the conclusion as proved by means of that premise.

it is usual to apply the title of “na?ve realism” to the theory which accepts as self-evident and indubitable the reality of the [300]world-picture which is immediately given to us. the opposite theory, which regards this world as merely the content of our consciousness, is called “transcendental idealism.” hence, we may sum up the outcome of the above discussion by saying, “transcendental idealism demonstrates its own truth, by employing the premises of the na?ve realism which it seeks to refute.” transcendental idealism is true, if na?ve realism is false. but the falsity of the latter is shown only by assuming it to be true. once we clearly realise this situation, we have no choice but to abandon this line of argument and to try another. but are we to trust to good luck, and experiment about until we hit by accident upon the right line? this is eduard von hartmann’s view when he believes himself to have shown the validity of his own epistemological standpoint, on the ground that his theory explains the phenomena whereas its rivals do not. according to his view, the several philosophical systems are engaged in a sort of struggle for existence in which the fittest is ultimately accepted as victor. but this method appears to us to be unsuitable, if only for the reason that there may well be several hypotheses which explain the phenomena equally satisfactorily. hence, we had better keep to the above line of thought for the refutation of na?ve realism, and see where precisely its deficiency lies. for, after all, na?ve realism is the view from which we all start out. for this reason alone it is advisable [301]to begin by setting it right. when we have once understood why it must be defective, we shall be led upon the right path with far greater certainty than if we proceed simply at haphazard.

the subjectivism which we have sketched above is the result of the elaboration of certain facts by thought. thus, it takes for granted that, from given facts as starting-point, we can by consistent thinking, i.e., by logical combination of certain observations, gain correct conclusions. but our right thus to employ our thinking remains unexamined. there, precisely, lies the weakness of this method. whereas na?ve realism starts from the unexamined assumption that the contents of our perceptual experience have objective reality, the idealism just described starts from the no less unexamined conviction that by the use of thought we can reach conclusions which are scientifically valid. in contrast to na?ve realism, we may call this point of view “na?ve rationalism.” in order to justify this term, it may be well to insert here a brief comment on the concept of the “na?ve.” a. d?ring, in his essay über den begriff des naiven realismus,10 attempts a more precise determination of this concept. he says, “the concept of the na?ve marks as it were the zero-point on the scale of our reflection upon our own activity. in content the na?ve may well coincide with the true, for, although the [302]na?ve is unreflecting and, therefore, uncritical or a-critical, yet this lack of reflection and criticism excludes only the objective assurance of truth. it implies the possibility and the danger of error, but it does not imply the necessity of error. there are na?ve modes of feeling and willing as there are na?ve modes of apprehending and thinking, in the widest sense of the latter term. further, there are na?ve modes of expressing these inward states in contrast with their repression or modification through consideration for others and through reflection. na?ve activity is not influenced, at least not consciously, by tradition, education, or imposed rule. it is in all spheres (as its root nativus, brings out), unconscious, impulsive, instinctive, d?monic activity.” starting from this account, we will try to determine the concept of the na?ve still more precisely. in every activity we may consider two aspects—the activity itself and our consciousness of its conformity to a law. we may be wholly absorbed in the former, without caring at all for the latter. the artist is in this position, who does not know in reflective form the laws of his creative activity but yet practises these laws by feeling and sense. we call him “na?ve.” but there is a kind of self-observation which inquires into the laws of one’s own activity and which replaces the na?ve attitude, just described, by the consciousness of knowing exactly the scope and justification of all one does. this we will call “critical.” [303]this account seems to us best to hit off the meaning of this concept which, more or less clearly understood, has since kant acquired citizen-rights in the world of philosophy. critical reflection is, thus, the opposite of na?ve consciousness. we call an attitude “critical” which makes itself master of the laws of its own activity in order to know how far it can rely on them and what are their limits. theory of knowledge can be nothing if not a critical science. its object is precisely the most subjective activity of man—knowing. what it aims at exhibiting is the laws to which knowing conforms. hence, the na?ve attitude is wholly excluded from this science. its claim to strength lies precisely in that it achieves what many minds, interested in practice rather than in theory, pride themselves on never having attempted, viz., “thinking about thought.”