What exists, let us call it X. Then X is whatever it is, but what any apparatus of cognition (for example, a human mind) will cognize it to be, will also depend on that apparatus of cognition. No apparatus of cognition sees what is actually existing as it really is but only from its own unique perspective and limitations. Because all apparatuses are more or less different from one another to different degrees (they can be similar but can never be the same), so X will be cognized differently by different cognizers; and no one will know what X is . That is the relation of all cognizers to what is actually existing or happening.
The crucial point here is to remember that whatever is actually existing is X and not a table, a chair, an atom or a molecule etc. because a chair or molecule etc. is what results after the cognitive activity of that particular cognizer from his own unique perspective which is different in varying degrees from the perspectives of all other cognizers in the universe.
The reason why human beings in everyday life seem to have similar views of tables, chairs and mountains etc. is that because their cognitive apparatuses are similar enough to each other to reach at a working consensus. This may not be so between different species or between proto humans, humans and post humans or between humans and some other unknown kinds of life in the universe.
Anything which anyone can know is relative to him and from his unique perspective. All knowing is perspectival and anything which humans know is known only from human perspective. The universe which human scientists know is human universe and not what actually exists or is actually happening.
So the huge mistake in common thinking is this: When someone perceives “a chair”, he (the knower, perceiver or cognizer etc.) jumps to the conclusion that what exists there is a chair and that the chair has an absolute existence of its own independently of him and that he is a mere spectator of the scene and has no part in making the scene.