Eckard
"May I understand your "closed system" as human's ultimate mental restriction to input via the sensory systems?"
No. We, and indeed all sentient organisms, are part of reality. We are not separate from it, as if we were, somehow, 'looking in'. So our very existence is the closed system. We cannot transcend that, and thereby know of anything extrinsic to it (assuming it exists anyway). But, although we are trapped in a sensory loop, within that, the sensory systems which enable that particular form of awareness in the first place, do not control/create what we are enabled to be aware of. That is, 'our' reality occurs independently of these processes. They receive physically existent phenomena.
It is the subsequent processing that is one of the problems. All the 'mental' stuff is just another 'nuisance', ie another factor which prevents the sensory systems from functioning perfectly by causing variation from the original. But all these factors are concerned with the mechanics of the processes, not metaphysical issues which cannot be resolved, by definition, and are irrelevant to an objective explanation of physical existence as it is detectable to us.
"There is no present time between past and future". Now, given that there is a physical existence, and it alters (ie occurs in different states), how can that statement be true? There must be a point of existence (the notion of time is irrelevant). Otherwise there is no physical existence, let alone something which can then occur differently! It cannot be what has occurred, neither can it be what has yet to occur. It can only be what is occurring. You are quite right about the vagueness of the quantities of time you quote. What is being referred to here is the deconstruction of physical existence until a physically existent state is 'revealed', ie that which had physical existence and involved no form of change. This probably revolves around the condition of the properties of the elementary parts. But that is what, to answer your question, is what existence means. This is not to be confused with the substance of existence.
"Such a practical judgment cannot be based on..." You are confusing knowledge of reality, with reality. It must have existed so that we can gain knowledge of it.
"Why do you imagine that the past must cease immediately". Because within any sequence of physical existence, two physically existent states cannot co-exist. The predecessor must cease for the successor to occur.
Without going into detail, and questioning the concepts, but just to convey the point. Two examples:
1 Take any type of elementary particle which is doing something. Now, in terms of substance, that is it, that is the 'bottom-line'. In terms of reality, ie what is existent at any time, that is not it. Because the question arises as to what constitutes a physically existent state? Say the 'doing something' was 'spinning'. Do we designate a physically existent state as half a spin, a whole spin, a million spins? No, all those options include change (ie must be more than one state). A physically existent state (ie a reality) in this circumstance will be one 'degree' of spin, ie where there is no further divisible state between two subsequent states. And a 'degree' would equate to the smallest particle in reality, ie the point at which no further spatial difference is possible. We have no chance of identifying this, I would suggest, but then the sensory systems evolved to give advantage in survival, not to perceive the very nature of our existence. But the whole point is, for physical existence to occur, and change, there must be a physically existent state of it, that must be definitive, there can only be one of them at a time in any given sequence, and it cannot involve change. This is the present, ie what at any given time is existent.
2 Say reality consisted of n differently shaped and coloured bricks, which have an innate property which caused them to move. Now, again the elementary substance of physical existence is the n bricks. The reality, ie what is existent, is a particular configuration of these as at any point in time, ie a particular physically existent state.
Finally, I must just stress that, generically, this is all very easy to say. What this is in terms of our reality is incredibly difficult. But these (and other) logical rules apply.
Paul
PS: I dumped the latest version of the first 22 paragraphs on my blog yesterday