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 any 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 Dasein. Being is given only if the understanding of Being, hence the Dasein, 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 Dasein. Ontology has for its fundamental discipline the analytic of the Dasein. 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 ontical―the Dasein. Ontology has an ontical 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 Dasein, the possibilities and destinies of philosophy are bound up with man's existence, and thus with temporality and with historicality, 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 ontical foundation* and the characterisation of this foundation itself.―from_The Basic Problems of Phenomenology_

~ Martin Heidegger

Thus with the question of the Being of truth and the necessity of presupposing it, just as with the question of the essence of knowledge, an 'ideal subject' has generally been posited. The motive for this, whether explicit or tacit, lies in the requirement that philosophy should have the '*a priori*' as its theme, rather than 'empirical facts' as such. There is some justification for this requirement, though it still needs to be grounded ontologically. Yet is this requirement satisfied by positing an 'ideal subject'? Is not such a subject *a fanciful idealization*? With such a conception have we not missed precisely the *a priori* character of that merely 'factual' subject, Dasein? Is it not an attribute of the *a priori* character of the factical subject (that is, an attribute of Dasein's facticity) that it is in the truth and in untruth equiprimordially?The ideas of a 'pure I' and of a 'consciousness in general' are so far from including the *a priori* character of 'actual' subjectivity that the ontological characters of Dasein's facticity and its state of being are either passed over or not seen at all. Rejection of a 'consciousness in general' does not signify that the *a priori* is negated, any more than the positing of an idealized subject guarantees that Dasein has an *a priori* character grounded upon fact.Both the contention that there are 'eternal truths' and the jumbling together of Dasein's phenomenally grounded 'ideality' with an idealized absolute subject, belong to those residues of Christian theology within philosophical problematics which have not as yet been radically extruded.The Being of truth is connected primordially with Dasein. And only because Dasein is as constituted by disclosedness (that is, by understanding), can anything like Being be understood; only so is it possible to understand Being.―from_Being and Time_. Translated by John Macquarrie & Edward Robinson, p. 272

~ Martin Heidegger