So, to return to the question of what "existence" means... Warning: this is going to be a very indulgent and overly serious post
I think it's one of the most primordial and purest questions of philosophy, and one that cannot be answered very easily. It's also probably one of the most beautiful questions that philosophy can ask and attempt to answer. Indeed, what does « being » mean ? I see this as not quite asking what it means to be a substance evolving over time through life and death (as in reincarnation) because the latter question, in a sense, already presupposes the meaning of being. « What does it mean to be ? » investigates the very fundament of the experience of being in a world, having sensations, thoughts and dispositions ; being (as a human) an entity that always exists in a certain relationship to what lies outside of it ; and perhaps, most crucially, being able to ask oneself the question about one's own existence : « why am I here rather than not here? ».
I think that, insofar as we are humans, the deep meaning of being can only be explored from a human perspective, because that is the only perspective we really experience in an unmediated way. Perhaps we can say: what distinguishes humans from other beings in the world is precisely this ability to ask the question, to wonder about the meaning of their own being. There is probably no definite anwer to this, as « being » is, at is most essential level, a phenomenon - not something that can be fit into a definition without losing all that is vital and alive about its deep meaning.
Among the different interpretations I have come across, Heidegger's has been the most captivating to me. He calls Dasein, or « being-there », the basic experience of the phenomenon of being in the world, and the existential constitution of human being. Dasein is defined by its thrownness, i.e. it is always existing from the perspective of having been thrown into a world which it didn't choose ; it has to make do with what is at its disposal. And it is always defined by its care, i.e. it fundamentally cares about other Dasein, which simply means that it is always projected onto other beings. Dasein makes no sense without the world in which it is thrown and the other beings with whom it interacts, or (in Heidegger's terminology) cares about.
From thrownness and care, the existence of Dasein as human being can begin to be endowed with meaning by specific individuals – but here we are already asking not what it means « to be », but the meaning of existence, that of particular or singular existences having to thrive in a particular kind of world. It is at this point that the question about the different shapes that existence can take, including the shape of an existence that dies and is reborn through reincarnation, can be asked. But when asking such a question, the meaning of the very word « existence » is already taken for granted, as well as the meaning of the word « world » against which existence is defined. Asking what shape existence through time can take implies understanding the meaning of the word « existence », what is meant when anybody says « I am ». Dasein, simply being-there, having been thrown into a world without deciding it, and being projected onto other beings as a way to thrive, is one possible answer.
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