On the other hand, it is also characteristic of the state of philosophical inquiry today and has been for a long time that, while there has been extensive controversy about whether or not the *a priori* can be known, it has never occurred to the protagonists to ask first what could really have been meant by the fact that a time-determination turns up here and why it must turn up at all. To be sure, as long as we orient ourselves toward the common concept of time we are at an impasse, and negatively it is no less than consistent to deny dogmatically that the *a priori* has anything to do with time. However, time in the sense commonly understood, which is our topic here, is indeed only one derivative, even if legitimate, of the original time, on which the Dasein's ontological constitution is based. *It is only by means of the Temporality of the understanding of Being that it can be explained why the ontological determinations of Being have the character of apriority*. We shall attempt to sketch this briefly, as far as it permits of being done along general lines.We have just seen that all comportment toward beings already understands Being, and not just incidentally: Being must necessarily be understood precursorily (pre-cecently). The possibility of comportment toward beings demands a precursory understanding of Being, and the possibility of the understanding of Being demands in its turn a precursory projection upon time. But where is the final stage of this demand for ever further precursory conditions? It is temporality itself as the basic constitution of the Dasein. Temporality, due to its horizonal-ecstatic nature, makes possible *at once* the understanding of Being and comportment toward beings; therefore, that which does the enabling as well as the enablings themselves, that is, the possibilities in the Kantian sense, are "temporal," that is to say, Temporal, in their specific interconnection. Because the original determinant of possibility, the origin of possibility itself, is time, time temporalizes itself as the absolutely earliest. *Time is earlier than any possible earlier* of whatever sort, because it is the basic condition for an earlier as such. And because time as the source of all enablings (possibilities) is the earliest, all possibilities as such in their possibility-making function have the character of the earlier. That is to say, they are *a priori*. But, from the fact that time is the earliest in the sense of being the possibility of every earlier and of every *a priori* foundational ordering, it does not follow that time is ontically the first being; nor does it follow that time is forever and eternal, quite apart from the impropriety of calling time a being at all.” ―from_The Basic Problems of Phenomenology_
Martin Heidegger
a-priori, being, heidegger, metaphysics, ontology