Ship of Theseus

If there are

Time-invariant parts in the

Ship of Theseus that are never replaced, they could be considered the physical essence of that entity. In humans, while skin cells and many atoms of organs are replaced through digestion processes, the core neuronal structure of the brain,

Cardiac muscle cells, and

Lens protein in the eyes are never replaced at the atomic level throughout one's lifetime. Thus, we can consider the physical essence of a human to be the core framework excluding

Neuroplasticity, heart muscle, and visual proteins.