What is money? We all assume we know, because money is a commonplace feature of everyday life. Money is what we earn and exchange for goods and services. Everyone thinks the money they're familiar with is the only possible system of money—until they run across an entirely different system of money.
Then they realize money is a social construct, a confluence of social consensus and political force-- what we agree to use as money, and what our government mandates we use as money under threat of punishment.
We assume that our monetary system is much like a Law of Nature: since it's ubiquitous, it must be the only possible system.
But there are no financial Laws of Nature for money. In the past, notched sticks served as money. In other non-Western cultures, giant stone disks (rai, a traditional form of money on the island of Yap) and even salt served as money.