Lange then entered on a career of militant journalism in the cause of political and social reform. He was also prominent in the affairs of his town, yet found leisure to write most of his best-known books, Die Leibesitbungen (1863), Die Arbeiterfrage (1865, 5th ed. 1894), Geschichte des Materialismus und Kritik seiner Bedeutung in der Gegenwart (1866; 7th ed. with biographical sketch by H Cohen, 1902; Eng. trans., EC Thomas, 1877), and JS Mill's Ansichten uber die sociale Frage (1866).
In 1866, discouraged by affairs in Germany, he moved to Winterthur, near Zürich, to become connected with the democratic newspaper, Winterthurer Landbote. In 1869 he was Privatdozent at Zürich, and next year professor. The strong French sympathies of the Swiss in the Franco-German War led to his speedy resignation. Thenceforward he gave up politics. In 1872 he accepted a professorship at Marburg. Unhappily, his vigorous frame was already stricken with disease, and, after a lingering illness, he died at Marburg, on the 23rd of November 1875, diligent to the end.
His Logische Studien was published by H Cohen in 1877 (2nd ed., 1894). His main work, the Geschichte des Materialismus, which is brilliantly written, with wide scientific knowledge and more sympathy with English thought than is usual in Germany, is rather a didactic exposition of principles than a history in the proper sense. Adopting the Kantian standpoint that we can know nothing but phenomena, Lange maintains that neither materialism nor any other metaphysical system has a valid claim to ultimate truth. For empirical phenomenal knowledge, however, which is all that man can look for, materialism with its exact scientific methods has done most valuable service. Ideal metaphysics, though they fail of the inner truth of things, have a value as the embodiment of high aspirations, in the same way as poetry and religion. In Lange's Logisthe Studien, which attempts a reconstruction of formal logic, the leading idea is that reasoning has validity in so far as it can be represented in terms of space. His Arbeiterf rage advocates an ill-defined form of socialism. It protests against contemporary industrial selfishness, and against the organization of industry on the Darwinian principle of struggle for existence.
See OA Ellissen, F. A. Lange (Leipzig, 1891), and in Monatsch. d. Conieniusgesell., 1894, 210 if.; H Cohen in Preuss. Jahrb. xxvii. 1876, 353 if.; Vaihinger, Hartzmann, Döhring und Lange (Iserlohn, 1876); JM Bösch, F. A. Lange und sein Standpunkt d. Ideals (Frauenteld, 1890); H. Braun, F. A. Lange, als Socialekonom (Halle, 1881).