"All philosophies, all science, all religions, inform us that this world of shapes, forms and name is but a phenomenal or shadow-world - a show-world - back of which rests Reality, called by some name of the teacher. But remember this, all philosophy that counts is based upon some form of monism - Oneness - whether the concept be a known or unknown god; an unknown or unknowable principle; a substance; an Energy, or Spirit. There is but one - there can be but One - such is the inevitable conclusion of the highest known human reason, intuition or faith."
Explaining the non-dualistic philosophy of Shankara (a.k.a. Samkara), Vedantist teacher, Swami Prabhavananada (in "The Spiritual Heritage of India") observes:
"The world, according to Samkara, 'is and is not.' Its fundamental unreality can be understood only in relation to the ultimate mystical experience, the experience of an illumined soul. When the illumined soul passes into transcendental consciousness, he realizes the Self (the Atman) as pure bliss and pure intelligence, the one without the second. In this state of consciousness, all perception of multiplicity ceases, there is no longer any sense of 'mine' and 'thine', the world as we know it has vanished. Then the Self shines forth as the One, the Truth, the Brahman, the basis of the apparent world."The following video serves as a brief introduction to the Advaita Vedanta, and introduces its many teachers, old and new, including Shankara as well as the late Ramana Maharshi and Sri Nisargadatta.
"The apparent world, as it is experienced in the waking state, may be likened, says Samkara, to an imagined snake which proves, on closer inspection, to be nothing but a coil of rope. When the truth is known, we are no longer deluded by the appearance - the snake-appearance vanishes into the reality of the rope, the world vanishes into Brahman."
[Swami Prabhavananda, "The Spiritual Heritage of India," pp. 283-284.]