One day is much like the next day; the weather may be different and a person may be different things in different places, but within your body one day feels much like the day before. Small differences happen to the place which the mind inhabits, and slowly these differences become significant, so that how a person looks, feels and thinks at fifteen is very different fifty years on.
But for all those changes there is an essential essence, which is not definable which remains constant. The youngster walking to school to sit examinations in the summer of 1964 has the same essence fifteen years on when he draws his old age pension.
Perhaps this essence is that part of the soul that protrudes into the conscious mind and directs it, when the conscious mind allows. It is beyond physical direction but can be subdued or restrained by thought. Let it roam free.