In our account of the fourth thesis we meet with a very central problem, one that is recurrently discussed in philosophy but only in a limited horizon the question of being in the sense of the "is," the copula in assertion, in the logos. The "is" has received this designation "copula" because of its combinatory position in the proposition intermediate between subject and predicate: S is P. Corresponding to the fundamental position in which the "is" occurs in the logos or assertion, and in conformity with the progress of the problem's development in ancient ontology, this "is" as copula was dealt with in the science of the logos, logic. Thus it came about that a very central and by no means arbitrary problem of being was forced aside into logic. We say "forced aside" because logic itself developed into a separate discipline within philosophy and because it became the discipline that most of all succumbed to induration and separation from the central problems of philosophy. It was Kant who first gave logic a central philosophical function again, though in part at the cost of ontology and above all without trying to rescue so-called academic logic from its philosophically alienated superficiality and vacuity. Even Hegel's more advanced attempt to conceive of logic as philosophy once again was more an elaboration of the traditional problems and stock of knowledge than a radical formulation of the problem of logic as such. The nineteenth century is not at all able to maintain itself at the level of Hegel's approach to the question but relapses into academic logic and, in fact, in such a way that questions of an epistemological and psychological nature get confused with specifically logical problems. Among the most significant treatments of logic in the nineteenth century, we may cite those of John Stuart Mill, Lotze, Sigwart, and Schuppe. Schuppe's epistemological logic receives much too little attention nowadays.