It is - Katha Upanishad

naiva vācā na manasā prāptum śakyō na cakṣuṣā  
astīti bruvato’nyatra katham  tadupalabhyatē 
Katha Upanishad, 6.12

Not by speech, nor by mind, nor by eye is it to be obtained.

How else can it be known, other than to tell itself, “It is!”