The conventional, linear depiction of time — at least as old as Newton, with philosophical roots reaching as far back as Aristotle — presents it as a straight line in which each passing moment recedes behind the present, just as each approaching moment arrives from a future stretched out in front of us along the time-line we are travelling. It is surprising how pervasive and apparently convincing this depiction is at first blush — given that it is simply not true to our experience of time at all. For the past exists for us as a whole, not strung out along a line: to retrieve a past moment from six weeks ago, we don’t have to rewind the entire chain of events to get there: we jump immediately to the last days of summer. And we can jump from there to any other past moments, without having to trace out or locate those moments on any linear time-lines. The past is, if you will, omni-present to itself. At least that’s the way it seems to us. But then the question becomes: is this true only of our experience of the past? — or is it true of the past itself
[…]
In other words, how do you get from phenomenology (or how things appear) to ontology and how things actually are? To be sure, past events co-exist in memory — we can scan the past and access this event or jump to that event, without having to replay the entire succession of moments between them. But how do we get from this psychological experience/recollection of the past to the notion that past events themselves co-exist ontologically? This is where Deleuze draws on Henri Bergson. The past for Bergson is not the repository of a linear series of passing presents, but an a-temporal bloc where each and every past event co-exists with all the others. For Bergson, it is not just in memory that one event can be connected with any other, irrespective of their respective places on a time-line: in the Bergsonian past, past events themselves co-exist, inhabiting a realm that Bergson calls the virtual: the past as a virtual whole […] (or as a bloc) is the condition for actual events to take place in the present, just as — for example — the language-system as a virtual whole (or what the structuralists call a structure, langue) is the condition for actual speech acts to take place in the present. This view of the past as a condition for the actualization of the present connects with the privileging of becoming over being that Deleuze adopts from Friedrich Nietzsche. Being is merely a momentary, subsidiary, and largely illusory suspension (or “contraction”) of becoming, according to this view; becoming is always primary and fundamental. This means not merely that each and every thing has a history — rather, each and every thing simply is its history: apparent being is always the temporary but actual culmination or expression of real becoming; it is the present actualization of antecedent conditions contained in the virtual past. In the terminology of A Thousand Plateaus, the process of actualization is called “stratification.”
-- Excerpt From: Eugene W. Holland. “Deleuze and Guattaris: A Thousand Plateaus.” iBooks.