Nothing fuzzy: money is debt. Ultimately modern money originates from a ledger entry which denotes an asset and a liability (a deposit and a loan; which is the asset and which is the liability depends on your perspective).
State issued currency is exactly the same. The liability from the perspective of the state is that central bank issued money can be used to satisfy a tax liability.
The relative value of things change even in the absence of money (e.g. in a barter economy, or for one person stuck alone on an island). Therefore the fuzziness cannot be attributed all to money.
State issued currency is exactly the same. The liability from the perspective of the state is that central bank issued money can be used to satisfy a tax liability.