“I am the body” - All animals know that. It's strange that many people around us also believe in the same concept!!
Exactly who am I? Who am I!? Yes! I am breathing. Without breathing I don't have a chance to stand. All day and night I breath in and out. Even when we are at sleep the breathing carries on at his own wish. While we are sleeping, the body exists, but there is no consciousness at the body. At that point of time the same person exist to call the body, 'I', but he does not accept the body as 'I'. That's why the body keep laying as a inert object. So by this logic it can be understood that I am not the body. When we get up from sleep, we again start broadcasting 'I', because other than the body there comes nothing in front.
So it can be concluded what we can't see but exist, is soul / 'Atma'. If that soul is not body, and if it's my true 'I', then if the body gets destroyed, that 'I' must never be destroyed.
We live our whole life with our constant companion, breathing, which we get by birth as our capital savings. We are worried to send it for a vacation, also for a few moment. When the breathing leaves by the rules of nature, the body won't go. The body dies. But if I am attached to the body always thinking about it, the consciousness resides in this body as 'I', of course have to die with the body. Yes!! Each and everyday people in this world, dies wailing like this.
Hence saints advised us to stay attached with breathing. So that we may leave with breathing and the true 'I' remains ever. Then we don't have to say, "I came alone in this world and will leave alone from this world." It will be like, "I will leave this world with whom I came, and with what I came."