Abstract
Jēkabs Osis (1860–1920) is the first academically educated Latvian philosopher and one of the founders of the University of Latvia. However, Osis never worked there. His academic life was closely tied with the University of Tartu, where he studied theology, philosophy and eventually became a professor of philosophy. Inspired by his mentor, professor of philosophy, Gustav Teichmüller, Osis turned his attention to the works of Leibniz, most notably those about the nature of substance. Osis aspired to unify the understanding of the notion of substance with the necessity to substantiate the immortality of individual entities. Osis criticized the philosophy of Leibniz and verified a new understanding of substance that is found in the works of Teichmüller.