In his V ergleichende Physiologie des Gesichtssinns (1826), Johannes Muller had developed the doctrine of the "specific energy" of sense organs ; he had shown that the quality of the individual sense data,—the constitution of colour, tone, smell, etc.—is not to be explained from the constitution of the external stimulus, but from the peculiarity of the organ that conveys the sensation. This result is taken up by Helmholtz in his lecture Ueber das Sehen des Menschen (1855), and interpreted as an empirical confirmation of that which Kant had determined by general a priori considerations. The true and permanent achieve ment of Kantian philosophy, according to this interpretation, con sists in having shown the participation of the innate laws of the mind in the formation of our ideas.
This interpretation was accepted by contemporary philosophers, especially by Friedrich Albert Lange (1828-1875). Lange, in his History of Materialism (1866), propounds as the essence of Kantian philosophy the proposition that what we call the "reality" of things is in truth nothing but their "appearance for the human species." Thus, the concept of causality, for instance, being rooted, according to this theory, in our psycho-physical organi zation, is prior to all experience, an a priori disposition of the human mind. Accordingly, it has within the field of human expe rience unlimited validity ; but beyond this no significance what ever. By extending this interpretation to all parts of the Kantian system, Lange arrives at the conclusion that not only is the con cept of the "thing-in-itself" the "concept of a perfectly prob lematical something" having significance only as a "limiting term" (Grenzbegriff), but that even the "intelligible world," which was used by Kant as the foundation of ethics, is a "world of poetry." This poetry, to be sure, is, according to Lange, "a necessary fruit of the mind, issuing from the inner and most vital roots of the species." But in this very implication, which threat ened to transform the Kantian transcendental idealism into a fictionism of the type developed later by Vaihinger, the deficien cies of the empirico-physiological interpretation of Kantian Apriorism became clearly apparent.
which Eduard Zeller had delivered in the year 1862 as an intro duction to his course on logic and epistemology in Heidelberg, he had pointed out that epistemology formed the formal basis of philosophy as a whole, that it was epistemology "from which the final decision on the correct method in philosophy and science generally had to come." Hegel's Wissenschaft der Logik and his Phaenomenologie des Geistes had been the last grandiose attempts to comprise the whole of knowledge in its content and to develop it constructively from one unifying idea. Zeller tries to show that the attempt did not reach its goal and could not reach it, "because it overlooks the conditions of human knowledge, for it purports to grasp with one swoop from above the ideal of knowledge which, in reality, we can approach only gradually through complicated labour from below." But the magic circle of the Hegelian system—so he says—will not allow itself to be broken, so long as the presuppositions of the latter are not investi gated anew and more thoroughly than before; and this very investigation necessarily leads back to Kant.
What is here expressed as a purely programmatic idea, Otto Liebmann (184o-1912) tried to carry out in his main philosoph ical writings. The very first of Liebmann's writings, Kant und die Epigonen (1865), attempts to show that the successors of Kant all missed the way which he had clearly recognized and indicated. In an intensive criticism of the "idealistic," the "realistic" and the "empirical" tendencies of post-Kantian philosophy, Liebmann tries to point out that all these movements—the systems of Fichte, Schelling and Hegel, as well as the systems of Herbart, Fries and Schopenhauer—suffer from a common fault. They all assign to the concept of the "Absolute" or of the "thing-in-itself" a central place and make it the fundamental concept of meta physics, whereas Kant's doctrine, if rightly understood and fur ther developed, implies the very opposite, namely, that this con cept is a non-concept, that all cognition moves within the realm of mere relationships, but can never grasp or positively deter mine an "Absolute." Hermann Cohen.—However, it was only in Hermann Cohen (1842-1918) that neo-Kantianism reached its climax. In his three great works on Kant : Kants Theorie der Erfahrung (1871, 2nd ed. 1885); Kants Begrundung der Ethik (1877, 2nd ed. 191o); Kants BegrUndung der Aesthetik (1888), Cohen gave for the first time a critical interpretation of the entire Kantian system which, with all its penetration into the specific detail of Kant's funda mental doctrines, sets, nevertheless, one single systematic idea into the centre of the investigation. This idea is that of the "transcendental method." From Friedrich Albert Lange, with whom Cohen was closely connected by ties of personal friendship, Cohen differs especially in that he rejects any psychological interpretation of Kant's Apriorism, any explanation of the a priori by the "psycho-physical organization" of man.