What is the Presence of Absence?

What does it mean for a thing not to exist, for it to be quite literally “conspicuous by its absence”?

When a thing that is there is removed, it is clearly an absence marked by memory or by material evidence, not that memory is perhaps all that trustworthy and notwithstanding that it is an irreducible component and foundation for the structure and grammar of cognition – we could hardly think at all without it. Memory, curiously, is always of a moment that is now absent, missing, gone. You can not (and a negation such as “not” is itself a kind of logical absence) remember the present but your future self – themselves an absence to the present – can do so, but that future self is also absent, although you are travelling inexorably towards becoming them.

When a sound is silenced, we notice the difference and this – in effect – is the bearer and definition of information as difference, not peculiar or unique to sound and silence but useful enough to carry a concept.

That is a start: the absence of a thing is still, in fact and almost counter-intuitively, the presence of something. It is the presence of that difference by which we might define information.

In this way, an absence is itself a thing and regardless that this absence might be a person, a place, an object, another moment in time (or memory – itself perhaps and effectively just another place), of the cessation of life, of the unrelenting tides of unexpected events that the Universe is bound to rain down upon us, of the concept of our own (mortal) absence or of the negation to any positive assertion of fact, truth or reality.

What is the presence of absence? I may as well ask “what does it mean to not know a thing?” as in essence the presence of absence is the existence of unknowing.

How can we not know a thing when knowledge appears as always reflexively built and orbiting in some sense around it’s Object?

There appears to be a fundamental ontological (as much as epistemological) principle at work here. What is the nature of a foundational information (and entropy) asymmetry in our world, in our experience?