Human understanding, ordinary intelligence is of no use here, and a lot of learning founded on this understanding, also, is of not much help. The nature of reality becomes a difficulty for the human understanding because of there being no defining characteristics of reality. You cannot say it has a colour. You cannot say it has a shape. You cannot say it has any kind of quality which can be interpreted in human language.
All definition is in terms of visible or sensible characters.
The sensible character of an object is not the ultimate definition of it, because we are here trying to understand the essential constituent of an object and not its character as it is presented to the senses. The test of reality, the nature of Truth, or Satya, is non-contradiction. Truth is that which can never be contradicted by any other definition, experience or realisation, which means to say that eternity is the character of Truth. Nothing in this world can be said to be ultimately real, because everything passes into something else. The whole world is transitory.