Adi Shankaracharya, in the Dakshinamurthy Stotram, says --

विश्वं दर्पणदृश्यमाननगरीतुल्यं निजान्तर्गतं

पश्यन्नात्मनि मायया बहिरिवोद्भूतं यथा निद्रया ।

यः साक्षात्कुरुते प्रबोधसमये स्वात्मानमेवाद्वयं

तस्मै श्रीगुरुमूर्तये नम इदं श्रीदक्षिणामूर्तये ॥१॥

Vishvam Darpanna-Drshyamaana-Nagarii-Tulyam Nija-Antargatam

Pashyann-Aatmani Maayayaa Bahir-Ivo[a-U]dbhuutam Yathaa Nidrayaa |

Yah Saakssaat-Kurute Prabodha-Samaye Sva-[A]atmaanam-Eva-Advayam

Tasmai Shrii-Guru-Muurtaye Nama Idam Shrii-Dakssinnaamuurtaye ||1||

Meaning:

(Salutations to Sri Dakshinamurthy Who Awakens the Glory of the Atman within us through His Profound Silence)

1.1: The Entire World is Like a City Seen within a Mirror, the Seeing happening within One's Own Being,

1.2: It is a Witnessing happening within the Atman, (the Witnessing) of the Externally Projected World; Projected by the Power of Maya; As if a Dream in Sleep,

1.3: One Experiences this Directly (this Play of Maya) during Spiritual Awakening within the Non-Dual Expanse of One's Own Atman,

1.4: Salutations to Him, the Personification of Our Inner Guru Who Awakens This Knowledge through His Profound Silence; Salutation to Sri Dakshinamurthy.

This a direct experience and can be realized very easily. Anything we can experience in the world of objects is only with our consciousness. Swami Sarvapriyananda says there are three steps --

  1. There is the world of objects and there is my consciousness.

  2. I experience the world of objects with my consciousness .

  3. I realize that these objects are only known in my consciousness, there is no way to know if they exist independent of my consciousness.

WRT the 3rd point, someone might argue that, "Sure we can. Ask another person to check if the object exists. They confirm it and that's how we know if it has independent existence apart from consciousness!"

That is a logical mistake. Why? Here's why. Whether you know something directly using your senses and mind, or you know something indirectly, via testimony of "others", all knowing is only in your own consciousness. Therefore, all things that are known are only in your own consciousness, never outside it.

References