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............