The question of what comprises an external reality is ancient. The frustrated physicist cries “shut up and measure”; and he is right for all that we can know is only that which we can measure. The rest, we cannot know. The rest is the unknowable. We can only describe the knowable, and so we do through science, and sometimes through art. Optimists amongst us contend that we could also describe the relationship between the knowable and the unknowable. I think that we can do so only as a conjecture. Let’s call it “the objective world conjecture”. Our favorite tool here is logical abstractions; that is; thinking about non-objects. In the abstract, therefore, we can assume that there exists U, the unknown. And the K – the known – is, somehow (via the mysterious chance mechanisms of evolution perhaps), configured within U. This is a conjecture that, alas, we can never prove. The objective world will be forever unknowable. The reason for this should be obvious by the definition we give to U; K is always a subset of U; K+K1 ,where K1 is a new discovery, is also a subset of U and so on, for any Ki, ad infinitum.
From this U-neverland arise the ghosts of our measurements, the contents of our consciousness, and the K-objects of our senses, emotions and feelings. When I look at a tree I see the only thing that can be seen. I call it a “tree” and, if I utter the word in any language, or draw a “tree”, the overwhelming majority of my species will intuitively imagine a “tree”, different in its details but similar in its essence. Our “essential tree” is an object of K made up of things we decide to call “cells” and “atoms” and so on.
Interestingly, as we expand through scientific observation the limits of K into the vast (conjecturally) expanse of U, we arrive at logical paradoxes. The objective world conjecture is paradoxical in itself, since we assume an increasing infinite progression of Knowledge (ΣΚi) which forever remains a subset of U.
The more abstract our reasoning the more paradoxical it becomes. And we thus arrive at the ultimate walls of K. When classical “objects” fade into quantum ghosts, our “K-trees” become U-trees, things of the unknowable; and we are fenced inside the realm of subjectivity, the only reality knowable.