Today, we're accustomed to thinking of small greenish paper rectangles as the definition of money, and we think of the US government as the only source of money. To honestly discuss sound money, we need to realize where our current money customs came from.
At first, it was every man for himself. You ate or wore what you could pick or catch.
Barter was the first advance. If you had some extra meat, and your neighbor had an extra fur, you might make a direct exchange. If food, water, clothing, and simple tools are the only goods on the market, barter is fine – you can always find someone who has what you want and wants what you have.