Kena Upanishad

“The Kena Upanishad approaches a more restricted problem, starts with a more precise and narrow inquiry. It concerns itself only with the relation of mind-consciousness to Brahmanconsciousness and does not stray outside the strict boundaries of its subject. The material world and the physical life are taken for granted, they are hardly mentioned. But the material world and the physical life exist for us only by virtue of our internal self and our internal life. According as our mental instruments represent to us the external world, according as our vital force in obedience to the mind deals with its impacts and objects, so will be our outward life and existence. The world is for us, not fundamentally but practically at any rate, what our mind and senses declare it to be; life is what our mentality or at least our half-mentalised vital being determines that it shall become. The question is asked by the Upanishad, what then are these mental instruments? what is this mental life which uses the external? Are they the last witnesses, the supreme and final power? Are mind and life and body all or is this human existence only a veil of something greater, mightier, more remote and profound than itself?”

Sri Aurobindo, Kena Upanishad