Siebe wrote: “You ask: Can you explain what else is there in a human being or any other living being?
It is the unconditioned element, Nibbana. Nibbana is always present,…”
There are four ultimate realities (paramatthathō):
Thoughts (citta)
Thought qualities or mental factors (cētasika)
Matter (rūpa)
Nibbāna
The first three belong to “this world”. Five aggregates encompass those.
Nibbana does not belong belong to “this world”.
Therefore, Nibbana cannot be a part of any living being. Even an Arahant gets to “full Nibbana” or Parinibbana only after the death of the physical body.