If we are indeed our body made of flesh and blood then our sense of self also should feel aged and dying. But we never feel aged or dying at any point in our life. We may feel depressed or anxious experiencing our old bodies but our sense of self never becomes "old". In that sense there is a contradiction between a feeling of a never changing self versus changing body. Therefore by deduction, Krishna concludes that the "self" is not the same as the "body". The soul (self) within the body occupies different bodies such as boy body, youth body, and old man body. Modern science interprets the same phenomenon as one person growing old. But that cannot be true as the "person" never feels old or growing as it is the body that ages and not the person.
Below is a series of pictures taken every five years by five individuals in the same spot with the same body language. They have been doing this for the past three decades. The pictures illustrates the changing body but the persons are the same - "The seven pictures to date are framed and hang on the wall of the cabin. The men still listen to the same '80s music they were listening to when they were 19 years old"
- editor CNN.
Bodies grow but sense of being does not. Below is a vivid example. As time passes on, persons will be missing from the picture and onto other bodies. This is the law of nature as spoken by Krishna.