A private key is a mathematical proof of ownership for whatever bitcoins are associated with that key. That mathematical proof is the only way to interact with those coins, nothing else can interact with it, no government, company or person can do it without those private keys. Cryptography makes it impossible.

Mathematics, are tangible parts of our universe. Numbers and notations might be abstract, since we gave them value and order, but mathematics are natural, and exist within the realm of the tangible universe.

So bitcoin is a currency based of tangible principles.
Fiat is abstract, since there is no protocol behind it. Your loans do not follow any mathematical immutable rule.

Fiat is a scam.