[4] Though the progress of his psychological thought compelled him to abandon the positive theology of the Moravians, he retained an appreciation of its spiritual or symbolic significance.

[4] Fries' most important treatise, the Neue oder anthropologische Kritik der Vernunft (2nd ed., 1828–1831), was an attempt to give a new foundation of psychological analysis to the critical philosophy of Immanuel Kant.

In 1816 he had published his views in a brochure, Von deutschem Bund und deutscher Staatsverfassung, dedicated to "the youth of Germany", and his influence gave a powerful impetus to the agitation which led in 1819 to the issue of the Carlsbad Decrees by the representatives of the German governments.

He was condemned by the Mainz Commission; the Grand Duke of Weimar was compelled to deprive him of his professorship; and he was forbidden to lecture on philosophy.

The grand duke, however, continued to pay him his stipend, and in 1824 he was recalled to Jena as professor of mathematics and physics, receiving permission also to lecture on philosophy in his own rooms to a select number of students.