The “thing-in-itself” is Immanuel Kant’s term for things as they exist apart from their existence as images in the mind of the observer.  Similarly, a “noumenon” is a posited object or event that exists independently of human sense and perception.  A “noumemon” cannot be perceived by human senses.  It is unknowable.  That does not mean it does not exist.  The noumenon world may be the driver of everything in our phenomenal world.

We can only think about what we can perceive.  Nonetheless, the noumenon world may display itself to the senses through art, particularly music (Schopenhauer’s assertion).  It may be perceived in meditation. Any display of this nature can be criticized as a delusion.