The feeling 'I am' is dormant at birth, it appears spontaneously say around the age of three. It is the essence of the five elements that make up the body or the food body. The body is a limitation, and as long as the 'I am' identifies itself with the body there is no chance of freedom, and death is certain. Eternity or immortality is possible only when you are free from the 'I am'. For this freedom to accrue you have to be after the 'I am', understand it, abide in it and transcend it. Judging from the enormous amount of spiritual literature available, the understanding, abidance and transcendence of the 'I am' appears to be too simple and crude a 'Sadhana' or practice, yet it works!