Physical perception is a perception of contrast; a contrast of light and shadow. All objects of perception are finite bodies, illuminated by light. Where there is only pure light, and nothing illuminated by it, there is no physical perception. The Guru is pure light.
For the physical eye to see, objects need to be illuminated with light, and there also has to be a light within that animates the eye and which is transfigured into the shape of the illuminated object. Thus, what actually is, an eye for the eye, which is also the ear behind the ear, the mind behind the mind, and the value that evaluates everything. This is Self.
The Guru is none other than the Self which resides in the heart of all. Thus, it is nearer than anything else and yet seems to be far, difficult to be approached and hardly known to anyone. We are so excited by what is seen that we even forget to ask who sees. Only one who has awakened from the hypnosis of sensuous life can recall the original and eternal verity of the ever-luminous Self. He will be able to arouse others from their intoxication of ignorance. One who can play this noble role among individual beings is called a guru for the simple reason that he is a Knower of the Guru.
According to Sankara, the Guru is like a burning coal lying buried in the ashes. Buried in the ashes lies a source of energy that can blow up into a conflagration and burn away all the dross of ignorance Sankara further describes the Guru as a peaceful person with such sympathy that he embraces all in his loving compassion for no reason whatsoever. Such a wise and kind guru who goes from place to place like a rain cloud, showering his compassion on all and greening the spiritual vitality of people yearning for wisdom, is compared to the spring season that gently and silently spreads its beneficence around the globe, bringing rejoicements to all those who have the refined sensibility to appreciate higher values.
~ Teachings of Guru Nithya Chaithanya Yati
Nitya Chaitanya Yati was a philosopher, psychologist, author and poet. He was an exponent of Advaita Vedanta and the successor to Nataraja Guru, who was in turn, the successor to Narayana Guru. You may continue to read more about him here