He contributed articles and reviews to the "Philosophisches Journal" of Fichte and Friedrich Immanuel Niethammer, and threw himself into the study of physical and medical science. In 1795 Schelling published "Philosophische Briefe über Dogmatismus und Kritizismus" ("Philosophical Letters on Dogmatism and Criticism"), consisting of 10 letters addressed to an unknown interlocutor that presented both a defense and critique of the Kantian system. In the period 1796/97 there was written the seminal manuscript now known as the "Das älteste Systemprogramm des deutschen Idealismus" ("The Oldest Systematic Program of German Idealism"). It survives in Hegel's handwriting. On its first publication (1916) by Franz Rosenzweig, it was attributed to Schelling. It has also been claimed for Hegel and Hölderlin. In 1797 Schelling published the essay "Neue Deduction des Naturrechts" ("New Deduction of Natural Law"), which anticipated Fichte's treatment of the topic in the "Grundlage des Naturrechts" ("Foundations of Natural Law"). His studies of physical science bore fruit in the "Ideen zu einer Philosophie der Natur" ("Ideas Concerning a Philosophy of Nature", 1797), and the treatise "Von der Weltseele" ("On the World-Soul", 1798). In "Ideen" Schelling referred to Leibniz and quoted from his "Monadology". He held Leibniz in high regard because of his view of nature during his natural philosophy period. In 1800 Schelling published "System des transcendentalen Idealismus" ("System of Transcendental Idealism"). In this book Schelling described transcendental philosophy and nature philosophy as complementary to one another. Fichte reacted by stating that Schelling was working on the basis of a false philosophical principle: in Fichte's theory nature as Not-Self ("Nicht-Ich" = object) could not be a subject of philosophy, whose essential content is the subjective activity of the human intellect. The breach became unrecoverable in 1801, after Schelling published "Darstellung des Systems meiner Philosophie" ("Presentation of My System of Philosophy"). Fichte thought this title absurd, since in his opinion philosophy could not be personalized. Moreover, in this book Schelling publicly expressed his estimation of Spinoza, whose work Fichte had repudiated as dogmatism, and declared that nature and spirit differ only in their quantity, but are essentially identical ("Identität").