The theme of the Mandukya Upanishad is an exposition of the Mystic Syllable, Om, with a view to training the mind in meditation, for the purpose of achieving freedom, gradually, so that the individual soul is attuned to the Ultimate Reality.
The basis of this meditation is explained in the Vidya (meditation), known as the Vaisvanara Vidya. This is the secret of the knowledge of the Universal Being, designated as Vaisvanara. Its simple form of understanding is a transference of human attributes to the Divine Existence, and vice versa. In this meditation, one contemplates the Cosmos as one’s Body. Instead of one contemplating oneself as the individual body, one contemplates oneself as the Universal Body. Instead of the right eye, there is the sun. Instead of the left eye, there is the moon. Instead of the feet, there is the earth. Instead of the head, there is the heaven, and so on. The limbs of the Cosmic Person are identified with cosmic elements, and vice versa, so that there is nothing in the cosmos which does not form an organic part of the Body of the Virat, or Vaisvanara. When you see the vast world before you, you behold a part of your own Body. When you look at the sun, you behold your own eye. When you look above into the heavens, you are seeing your own head. When you see all people moving about, you behold the various parts of your own personality. The vast wind is your breath. All your actions are cosmic movements. Anything that moves, does so on account of your movement. Your breath is the Cosmic Vital Force. Your intelligence is the Cosmic Intelligence. Your existence is Cosmic Existence. Your happiness is Cosmic Bliss.
Though the Mandukya Upanishad gives certain symbolic instances of identification of limbs with the Cosmic Body, the meditator, in fact, can choose any symbol or symbols for such form of identification.
The Vidya has its origin, actually, in the Rig-Veda, in a famous Sukta, or hymn, called the Purusha-Sukta. The Purusha-Sukta of the Rig-Veda commences by saying that all the heads, all the eyes, and all the feet that we see in this world are the heads, eyes, and feet of the Virat-Purusha, or the Cosmic Being. With one head, the Virat nods in silence; with another face He smiles; with a third one, He frowns; in one form, He sits; in another form, He moves; in one form, He is near; in another form, He is distant. So, all the forms, whatever they be, and all the movements and actions, processes and relations, become parts of the Cosmic Body, with which the Consciousness should be identified simultaneously. When you think, you think all things at the same time in all the ten directions; nay, in every way.
Excerpt from the Introduction to the Mandukya Upanishad by Swami Krishnananda