The whole idea of ethics is that it does not depend on anything unknowable, it does not teach anything unknown, but in the language of the Upanishad, "The God whom you worship as an unknown God, the same I preach unto thee." It is through the Self that you know anything. I see the chair; but to see the chair, I have first to perceive myself and then the chair. It is in and through the Self that the chair is perceived. It is in and through the Self that you are known to me, that the whole world is known to me; and therefore to say this Self is unknown is sheer nonsense. Take off the Self and the whole universe vanishes. In and through the Self all knowledge comes. Therefore it is the best known of all. It is yourself, that which you call I. You may wonder how this I of me can be the I of you. You may wonder how this limited I can be the unlimited Infinite, but it is so. The limited is a mere fiction. The Infinite has been covered up, as it were, and a little of It is manifesting as the I. Limitation can never come upon the unlimited; it is a fiction. The Self is known, therefore, to every one of us — man, woman, or child — and even to animals. Without knowing Him we can neither live nor move, nor have our being; without knowing this Lord of all, we cannot breathe or live a second. The God of the Vedanta is the most known of all and is not the outcome of imagination.
If this is not preaching a practical God, how else could you teach a practical God? Where is there a more practical God than He whom I see before me — a God omnipresent, in every being, more real than our senses? For you are He, the Omnipresent God Almighty, the Soul of your souls, and if I say you are not, I tell an untruth. I know it, whether at all times I realise it or not. He is the Oneness, the Unity of all, the Reality of all life and all existence.
Source: The Complete Works of Swami Vivekananda/Volume 2