When there is a perception of physical reality, the various parts of the brain show a change in neural activity as the neurons, which are the basic building blocks of the nervous system, fire in synchronicity. This results in the random visual patterns of light being perceived as specific shapes and forms. This is how the input from each of our senses is perceived as an experience of particular sound, touch, smell, taste or sight. If you now step back and look, you can see that before the observation or the perception, both the world as well as you, that is the seen and the seer, existed in a dynamic and chaotic state of activity. The act of perception changes that chaotic state into some sort of an order that becomes both the seen in the outside world and the process of seeing in the seer (as the activity in the nervous system) at the same time. At the moment of seeing, the probability of what is seen collapses into a specific form.
In the Bhagavad Gita, Sri Krishna, the Avatar and Master, chooses to give Arjuna the experience of Krishna’s cosmic form. Until then, Arjuna had seen Krishna only as the six foot tall, handsome friend. But upon Arjuna’s request, and after Krishna sees that Arjuna is ready to receive the cosmic form experience, He decides to give it to him. Arjuna then sees Krishna as the manifestation of the entire cosmos in all its seeming chaos. He cries out, ‘O Lord! I can see all the gods and deities in Your body. I can see all the sages and divine serpents. O Lord of the Universe, I see many arms, stomachs, faces, eyes and your limitless form. O Universal Form, I cannot see your beginning, middle or end.’ Arjuna is unable to bear the experience because he loses his intellectual control over things! He begs for equilibrium, meaning he wants his mind to be under his intellectual control again. He wants to know the cause and effect of things and feel that they are in his control, because the experience rips him away from his perception of the cause-and-effect logic itself.
As long as cause and effect logic is under your control, you feel you are perceiving reality! Your ego is strong and steady. But the moment the logic is shaken, you feel tremendous fear. You feel you are just a drop in an ocean. What Arjuna sees is the absolute Reality, but he is afraid of that very Reality because it doesn’t conform to his logic. He begs to revert to the earlier state of perception through his five senses alone, through his own solid identity, his own solid ego.
source: Living Enlightenment