Meditation (Dhyana)

Shri Devi said: One may meditate on a visible image, O Mahadeva. What is the nature of meditation on the invisible? Shri Shankara said: O Devi, sound, uttered by me, is the absolute. By pronouncing a mantra with a devoted mind, there is invisible meditation and so forth. Maheshvari, this is true, true, self evident, undoubtedly - Matrikabhedatantra XII, 5-7