Are we seeing the objects around us like plants, animals, rocks, etc because we have eyes? Are we hearing various sounds from the outside world due to our ears? In the same way touch-skin, odour – nose, taste – tongue pairs also should be considered. Common man may answer ‘yes’ to each of this question, and that is correct in certain sense, but only to reach in a more correct answer afterwards. In a deep analysis we may understand that ‘something’ is always supporting our all sensations. We cannot sense even if our sense organs are healthy and working properly. Why a sleeping man does not hear noises around him? Why he not waking up by the moderate touch of others? People may say he is in deep sleep. Yet after waking up, he knew that he was sleeping. He does not sense other’s touches and sounds while sleeping. But he is able to remember a dream that he saw after waking up. Who sensed dream while the man was sleeping? Why did the man not sense touch and sound of others in the way he sensed dreams? Is it his mind that dreamt? Then can the mind exist and function when all of our senses remain idle?
If we analyse the cognitions thoroughly, we can understand that the substratum of our sensations and mental states is nothing but our consciousness. Consciousness works with external organs and internal organ (mind) to sense the external and internal sensations. In our waking state the external organs sense the phenomenal world and pass the sensation to the internal organ (manas). Internal organ passes these sensations to the consciousness, one at a time, avoiding simultaneous perceptions. These are the steps to cognize an external object. In dream, mind conveys some memory impressions to the consciousness and in deep sleep no communication happens at all. Thus all cognitions depend on consciousness. We can’t cognize anything, mental or physical, without having consciousness. So what really exists? The external world or the consciousness?
Here a classification may be applied. On one side we have ‘consciousness’ and on the other, external objects. To sense the latter, we must need the former. In such a scenario, one may interpret the situation in the following manner. If ‘something’ depends on ‘another something’ for its existence/manifestation, or more clearly, if we sense a secondary entity (external world) only through a primary entity (consciousness), then the secondary entity can be said to be the manifestation or modification of the primary entity! In other words, when the existence of something depends solely on another thing, then the latter will have more validity and reality than the former. Then the secondary entity (external world) may not exist as an objective reality independent of the consciousness. Subjective consciousness itself can be said to be the objective external world. Perceiver and perceived is same, we can say. Subject - object classification is not there, ultimately.
This theory that ‘consciousness is the ultimate reality’ can be found prominently in the teachings of the Upanishads, Advaita Vedanta and Mahayana Buddhism.
 Though not simultaneous, it is said elsewhere that the transfer of perceptions from mind to the consciousness is so fast like a needle piercing each page of a book at a time, when we push the needle into the book.