Man's immediate certainty that there are real objects, which produce passive sensations, rests on faith

Friedrich Heinrich Jacobi

The point of departure for Jacobi's thought is the antinomy, or seeming contradiction, between realism and idealism. Baruch Spinoza was a dogmatic realist who drew out the logical consequences of the traditional definition of substance as that which is the cause of itself. According to this view, there could be only one substance, an infinite eternal being of which the world of nature is only a partial but determinate modification. The meaning of Spinoza's pantheism, or the identification of God with nature, was a subject of other disputes throughout the 19th century. Jacobi sided with those who thought that Spinoza was, in fact, an atheist who had reduced God to a logical, mathematical, and mechanistic concept of nature. Other writers and philosophers such as Johann Georg Hamann, Johann Gottfried von Herder, Lessing, and Mendelssohn held that Spinoza was the first religious thinker to seriously develop the philosophic dimensions of the concept of an infinite being. Largely through Jacobi's instigation the major figures of the Enlightenment produced an extensive literature of books, inquiries, and couterinquiries about Spinoza.
Jacobi saw in Spinoza the elimination of real subjectivity and in the philosophy of Immanuel Kant an opposite "nihilism of objects." Kant was the first to raise the critical question of how subjective consciousness arrives at a knowledge of things, and he concluded that ultimately we can know of things "only what we have placed in them." Thus for Kant, human experience is simply the appearance of the way things seem and are thought about according to the subjective conditions of the mind. Objects as things-in-themselves are unknowable.
The point of these criticisms was to show that if reason begins with objects it is unable to account for subjectivity and a subjective perspective annihilates objectivity. The conclusion which Jacobi drew was that the enterprise of human reason itself rests on faith. Man's immediate certainty that there are real objects, which produce passive sensations, rests on faith. And if the concept of objective nature depends on faith, then man's feelings and intuitions of freedom, moral principles, and religious certainties need not defer to rational skepticism.

Teachings
Jacobi's philosophy is essentially unsystematic. A fundamental view which underlies all his thinking is brought to bear in succession upon those systematic doctrines which appear to stand most sharply in contradiction to it, and any positive philosophic results are given only occasionally. The leading idea of the whole is that of the complete separation between understanding and apprehension of real fact. For Jacobi understanding, or the logical faculty, is purely formal or elaborative, and its results never transcend the given material supplied to it. From the basis of immediate experience or perception thought proceeds by comparison and abstraction, establishing connections among facts, but remaining in its nature mediate and finite. Wikipedia article "Friedrich Heinrich Jacobi". Read more