2. The reason Ultimate Reality cannot be grasped by thought or communicated in words is that thoughts and words, by definition, create distinctions and, hence, duality. Even the simple act of naming something creates duality because it distinguishes the thing that is named from all other things that are left unnamed. However, the mystics of all the great traditions agree that all distinctions are imaginary and that the Ultimate Nature of Reality is non-dual.

In essence things are not two but one. ...All duality is falsely imagined. Lankavatara Sutra (Buddhist)

No matter what a deluded man may think he is perceiving, he is really seeing Brahman and nothing else but Brahman. ...This universe, which is superimposed upon Brahman, is nothing but a name. Shankara (Hindu)

If we will see things truly, they are strangers to goodness, truth and everything that tolerates any distinction. They are intimates of the One that is bare of any kind of multiplicity and distinction. Meister Eckhart (Christian)

That Oneness is on the other side of descriptions and states. Nothing but duality enters speech's playing-field. Rumi (Muslim)

There all things are as one; Distinctions between "life" and "death," "land" and "sea," have lost their meaning. anonymous Hasidic master (Jewish)