First, a confession: several years ago, I wrote about the concept of "ontological priority" on here, and at the time, I thought I had invented the phrase. I knew that the phrase sounded like it could have originated with Martin Heidegger, and the term was inspired by his philosophical writing---but I thought it was my own. Lo and behold, several weeks ago, while reading the "Basic Writings" of Martin Heidegger, which includes the Introduction to Being and Time, I saw that he had actually used the phrase "ontological priority" there, and had used the words either apart or together in his explication of Dasein. Of course, my explanation is still better, so go read that before returning here!
First, a very brief introduction to the philosophy of Heidegger and a definition of "Dasein". Heidegger believed that Western philosophy had forgotten about "Sein", Being in it self, to focus on a series of "beings". Since "Sein" is a concept beyond any particular being, it was hard to explore or understand it. The process of Being trying to understand itself was "ontology", and the key to this, Heidegger wrote, was "Dasein", Being There, by which he meant human existence in the real world, only he didn't because when he said "anxiety", he wasn't talking about the emotion but something else. Look, it is complicated. Or else he was just pretentious and full of insistent terminology.
But importantly, right there in the introduction, Heidegger states that Dasein has ontological priority, and all other ontologies derive from it. What that means to me, is that all other forms of "ontology", meaning all other forms of understand being, must derive from our immanent experience of daily life. So, for example, all ontologies from a revealed religion, or a philosophical "natural religion", or from another philosophical viewpoint (such as a Spinozan geometric explication of human emotions), or maybe from a mystical liminal state, or any other way you might explore what it means to be---all of these, Heidegger insists, come second from the first ontology---which is, of course, the experience of daily life, as it is understood by a college professor in Germany with fascist tendencies. Only of course, daily life isn't really daily life, because you see, this is all very complicated.
Anyway, there are two problems with Heidegger's assertion that Dasein has ontological priority.
The first is that he never proves or demonstrates that this is the case, and never presents a convincing line of thought about why Dasein comes before any other type of ontology. Why exactly, does an immanent exploration of one's own being automatically make more sense than the understanding of one own's being from a first cause? Heidegger is not always great at explanations, but after reading back and forth, I couldn't seem to find even one of his vague poetic diatribes about why this was the case.
Secondly, and more important for me: peak or liminal experiences form a very important part of my own life. For me, this question of ontology is propelled, in various ways, through experiences of something beyond myself. And I also had questions about the wider, and not "daily" universe. Even as a young child, I always remember looking at the first pages of my books on dinosaurs, and wondering what came before that. And my interest wasn't confined to the natural sciences: I remember being 4 or 5 years old, and asking our pastor Whether angels were perfect?, which is quite a complex question, really. And most of my experience of "every day" life is still framed, deep down, in wanting to understand cosmology. I actually understand where the idea of "Dasein" is coming from, the richness of the immanent present, how every little texture of entering a new home or the indescribable subtly of a warm breeze in a park can communicate so much. Because those sensations are what life is about, and life means forever, and that is a mighty long time. But of course, there is something else. And after years or even decades of trying to understand how I could just merge into the moment, to feel everything around me totally, to have that warm glaze of confusion, that space cadet glow, I realized that on both technical grounds, and from my own experiences, that Heidegger's claim that the ontological priority belongs to daily life, or rather, to his version of daily life that isn't really daily life because it is complicated---well, it seems lacking.