The explicit objective behind sections 8 and 10 was to prevent having 13 (or 50, or whatever) independent currencies in each state, and the exchange-rate nightmare that comes with that. (Which is probably not that much of a nightmare today, when you can swipe a credit card through an ATM in Bangkok or Tokyo and have your bank account debited at the current exchange rate + a nominal exchange fee. Would’ve been a much bigger issue in 1789.)

Hence, the Constitution was written to allow two kinds of money:

– The U.S. dollar (whose value in terms of gold and silver is arbitrary and may be set by the Congress or its agent, or left to the market)

– Gold and silver coins.

At the time the Constitution was passed, paper money did not exist. (Hence, the word “coin.”) However, the Congress was authorized to regulate the value of the U.S. dollar. They could’ve ‘coined’ U.S. dollars out of copper and iron, if they wanted to. There’s no practical reason why they couldn’t have used paper.