The idealistic and the realistic stage of development to which we have now been attending, each ended with a one-sided result. Instead of actually and internally reconciling the opposition between thought and being, they both issued in denying the one or the other of these factors. Realism, on its side, had made matter absolute; and idealism, on its side, had endowed the empirical Ego with the same attribute—extremes in which philosophy was threatened with total destruction. It had, in fact, in Germany as in France, become merged in the most superficial popular philosophy. Then Kant arose, and brought again into one channel the two streams which, when separate from each other, threatened to lose themselves amid the sands. Kant is the great renovator of philosophy, who brought back to their point of divergence the one-sided efforts which had preceded him, and embraced them in their unity and totality. He stands in some special and fitting relation either antagonistic or harmonious to all others—to Locke no less than to Hume, to the Scottish philosophers no less than to the English and French moralists, to the philosophy of Leibnitz and of Wolff, as well as to the materialism of the French and the utilitarianism of the German clearing up period. His relation to the development of a partial idealism and a one-sided realism is thus stated: Empiricism had made the Ego purely passive and subordinate to the sensible external world—idealism had made it purely active, and given it a sovereignty over the sensible world; Kant attempted to strike a balance between these two claims, by affirming that the Ego as practical is free and autonomic, an unconditioned lawgiver for itself, while as theoretical it is receptive and conditioned by the phenomenal world; but at the same time the theoretical Ego contains the two sides within itself, for if, on the one side, empiricism may be justified upon the ground that the material and only field of all our knowledge is furnished by experience, so on the other side, rationalism may be justified on the ground that there is an apriori factor and basis to our knowledge, for in experience itself we make use of conceptions which are not furnished by experience, but are contained apriori in our understanding.

In order, now, that we may bring the very elaborate framework of the Kantian philosophy into a clearer outline, let us briefly glance at its fundamental conceptions, and notice its chief principles and results. Kant subjected the activity of the human mind in knowing, and the origin of our experience, to his critical investigation. Hence his philosophy is called critical philosophy, or criticism, because it aims to be essentially an examination of our faculty of knowledge; it is also called transcendental philosophy, since Kant calls the reflection of the reason upon its relation to the objective world, a transcendental reflection (transcendental must not be confounded with transcendent), or, in other words, a transcendental knowledge is one “which does not relate so much to objects of knowledge, as to our way of knowing them, so far as this is apriori possible.” The examination of the faculty of knowledge, which Kant attempts in his “Critick of Pure Reason,” shows the following results. All knowledge is a product of two factors, the knowing subject and the external world. Of these two factors, the latter furnishes our knowledge with experience, as the matter, and the former with the conceptions of the understanding, as the form, through which a connected knowledge, or a synthesis of our perceptions in a whole of experience first becomes possible. If there were no external world, then would there be no phenomena; if there were no understanding, then these phenomena, or perceptions, which are infinitely manifold, would never be brought into the unity of a notion, and thus no experience were possible. Thus, while intuitions without conceptions are blind, and conceptions without intuitions are empty, knowledge is a union of the two, since it requires that the form of conception should be filled with the matter of experience, and that the matter of experience should be apprehended in the net of the understanding’s conceptions. Nevertheless, we do not know things as they are in themselves. First, because the categories, or the forms of our understanding prevent. By bringing that which is given as the material of knowledge into our own conceptions as the form, there is manifestly a change in respect of the objects, which become thought of not as they are, but only as we apprehend them; they appear to us only as they are transmuted into categories. But besides this subjective addition, there is yet another. Secondly, we do not know things as they are in themselves, because even the intuitions which we bring within the form of the understanding’s conceptions, are not pure and uncolored, but are already penetrated by a subjective medium, namely, by the universal form of all objects of sense, space and time. Space and time are also subjective additions, forms of sensuous intuition, which are just as originally present in our minds as the fundamental conceptions or categories of our understanding. That which we would represent intuitively to ourselves we must place in space and time, for without these no intuition is possible. From this it follows that it is only phenomena which we know, and not things in themselves separate from space and time.

A superficial apprehension of these Kantian principles might lead one to suppose that Kant’s criticism did not essentially go beyond the standpoint of Locke’s empiricism. But such a supposition disappears upon a careful scrutiny. Kant was obliged to recognize with Hume that the conceptions, cause and effect, substance and attribute, and the other conceptions which the human understanding sees itself necessitated to think in the phenomena, and in which every one of its thoughts must be found, do not arise from any experience of the sense. For instance, when we become affected through different senses, and perceive a white color, a sweet taste, a rough surface, &c., and predicate all these of one thing, as a piece of sugar, there come from without only the plurality of sensations, while the conception of unity cannot come through sensation, but is a category or conception borne over to the sensations from the mind itself. But instead of denying, for this reason, the reality of these conceptions of the understanding, Kant took a step in advance, assigning a peculiar province to this activity of the understanding, and showing that these forms of thought thus furnished to the matter of experience are immanent laws of the human faculty of knowledge, the peculiar laws of the understanding’s operations, which may be obtained by a perfect analysis of our thinking activity. (Of these laws or conceptions there are twelve, viz., unity, plurality, totality; reality, negation, limitation; substantiality, causality, reciprocal action; possibility, actuality, and necessity.)

From what has been said we can see the three chief principles of the Kantian theory of knowledge:

1. We know only Phenomena and not Things in Themselves.—The experience furnished us by the external world becomes so adjusted and altered in its relations (for we apprehend it at first in the subjective framework of space and time, and then in the equally subjective forms of our understanding’s conceptions), that it no longer represents the thing itself in its original condition, pure and unmixed.

2. Nevertheless Experience is the only Province of our Knowledge, and there is no Science of the Unconditioned.—This follows of course, for since every knowledge is the product of the matter of experience, and the form of the understanding, and depends thus upon the co-working of the sensory and the understanding, then no knowledge is possible of objects for which one of these factors, experience, fails us; a knowledge alone from the understanding’s conceptions of the unconditioned is illusory since the sensory can show no unconditioned object corresponding to the conception. Hence the questions which Kant places at the head of his whole Critick; how are synthetical judgments apriori possible? i. e. can we widen our knowledge apriori, by thought alone, beyond the sensuous experience? is a knowledge of the supersensible possible? must be answered with an unconditional negative.

3. Still, if the human knowledge makes no effort to stride beyond the narrow limits of experience, i. e. to become transcendent, it involves itself in the greatest contradictions. The three ideas of the reason, the psychological, the cosmological, and the theological, viz. (a) the idea of an absolute subject, i. e. of the soul, or of immortality, (b) the idea of the world as a totality of all conditions and phenomena, (c) the idea of a most perfect being—are so wholly without application to the empirical actuality, are so truly regulative, and not constitutive principles, which are only the pure products of the reason, and are so entirely without a correspondent object in experience, that whenever they are applied to experience, i. e. become conceived of as actually existing objects, they lead to pure logical errors, to the most obvious paralogisms and sophisms. These errors, which are partly false conclusions and paralogisms, and partly unavoidable contradictions of the reason with itself, Kant undertook to show in reference to all the ideas of the reason. Take, e. g. the cosmological idea. Whenever the reason posits any transcendental expressions in reference to the universe, i. e. attempts to apply the forms of the finite to the infinite, it is at once evident that the antithesis of those expressions can be proved just as well as the thesis. The affirmation that the world has a beginning in time, and limits in space, can be proved as well as, and no better than its opposite, that the world has no beginning in time, and no spacial limits. Whence it follows that all speculative cosmology is an assumption by the reason. So also with the theological idea; it rests on bare logical paralogisms, and false conclusions, as Kant, with great acuteness, shows in reference to each of the proofs for the being of a God, which previous dogmatic philosophies had attempted. It is therefore impossible to prove and to conceive of the existence of a God as a Supreme Being, or of the soul as a real subject, or of a comprehending universe. The peculiar problems of metaphysics lie outside the province of philosophical knowledge.

Such is the negative part of the Kantian philosophy; its positive complement is found in the “Critick of the Practical Reason.” While the mind as theoretical and cognitive is wholly conditioned, and ruled by the objective and sensible world, and thus knowledge is only possible through intuition, yet as practical does it go wholly beyond the given (the sense impulse), and is determined only through the categorical imperative, and the moral law, which is itself, and is therefore free and autonomic; the ends which it pursues are those which itself, as moral spirit, places before itself; objects are no more its masters and lawgivers, to which it must yield if it would know the truth, but its servants, which it may use for its own ends in actualizing its moral law. While the theoretical mind is united to a world of sense and phenomena, a world obedient to necessary laws, the practical mind, by virtue of the freedom essential to it, by virtue of its direction towards an absolute aim, belongs to a purely intelligible and supersensible world. This is the practical idealism of Kant, from which he derives the three practical postulates of the immortality of the soul, moral freedom, and the being of a God, which, as theoretical truths, had been before denied.

With this brief sketch for our guidance, let us now pass on to a more extended exposition of the Kantian Philosophy.