Each moment we exist determines the previous moment we existed. Our degradation expresses evidence of its existence but perhaps not why it seems necessary, if indeed it is.
Stanley Wilkin I can see a possible rationale for saying that the present moment fixes the previous moment as past (e.g. by a kind of displacement?). A sense of necessity I might buy into is predicated on the idea that "to be is to be an event" (or a component of one). In other words, existence requires duration. Something can't exist only at a solitary mathematical point on a timeline; a finite interval of moments is needed for existence.1 That's also why some folks talk of "the specious present" instead of "the present moment".
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1 However, each such interval will contain infinitely many moments.
Karl, excellent elaboration but again although the past defines us and our actions it also goes against the concept of existence. If i exist most plausibly in the past, to what extent am I or do I have existence in the present? I always take or make actions in the past, but once any action is made that action and myself become redundant.