Our conduct of the ontological investigation in the first and second parts opens up for us at the same time a view of the way in which these phenomenological investigations proceed. This raises the question of the character of method in ontology. Thus, we come to the third part of the course: the scientific method of ontology and the idea of phenomenology. The method of ontology, that is, of philosophy in general, is distinguished by the fact that ontology has nothing in common with any method of the other sciences, all of which as positive sciences deal with beings. On the other hand, it is precisely the analysis of the truth-character of Being which shows that Being also is, as it were, based in a being, namely, in the Casein. Being is given only if the understanding of Being, hence the Casein, exists. This being accordingly lays claim to a distinctive priority in ontological inquiry. It makes itself manifest in all discussions of the basic problems of ontology and above all in the fundamental question of the meaning of Being in general. The elaboration of this question and its answer requires a general analytic of the Casein. Ontology has for its fundamental discipline the analytic of the Casein. This implies at the same time that ontology cannot be established in a purely ontological manner. Its possibility is referred back to a being, that is, to something optical―the Casein. Ontology has an optical foundation, a fact which is manifest over and over again in the history of philosophy down to the present. For example, it is expressed as early as Aristotle's dictum that the first science, the science of Being, is theology. As the work of the freedom of the human Casein, the possibilities and destinies of philosophy are bound up with man's existence, and thus with temporality and with historically, and indeed in a more original sense than is any other science. Consequently, in clarifying the scientific character of ontology, *the first task is the demonstration of its optical foundation* and the characterization of this foundation itself."―from_The Basic Problems of Phenomenology_