Since every living entity is an individual soul, each is changing his body every moment, manifesting sometimes as a child, sometimes as a youth, and sometimes as an old man. Yet the same spirit soul is there and does not undergo any change. That is you who is observing without change. This individual soul finally changes the body at death and transmigrates to another body – either material or spiritual.
The body is just like a machine, and according to the living entity's karma, he is offered a particular type of machine to move here and there under the control of material nature. For instance, by contact with the living entity, the material body develops into a working machine. Men with a poor fund of knowledge mistake the bodily machinery to be the living being, but the fact is that the living being is the basis of the bodily machine. The bodily machine is useless as soon as the living spark is away from it.
The bodily sufferings will continue until one surrenders to the Supreme Personality of Godhead. Until he surrenders, the conditioned soul is carried from life to life by the arrangement of material nature.