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Money is any asset widely accepted as payment. It serves three functions: medium of exchange (eliminates the double coincidence of wants problem of barter), unit of account (a common standard for quoting prices), and store of value (a way to transfer purchasing power over time). Forms of money include commodity money (intrinsic value), representative money (backed by a commodity), and fiat money (value by government decree). Economists measure the money supply using aggregates M1 (most liquid: currency + demand deposits) and M2 (M1 + savings + small time deposits).
Trace the evolution from barter to commodity money to fiat money using historical examples (gold standard, Bretton Woods, modern dollar). Categorize common assets (cash, savings accounts, Treasury bills, stocks) by their M1/M2 classification.
Start from what you know about scarcity and opportunity cost: people trade because they can gain from specialization. But direct barter — trading goods for goods — requires a double coincidence of wants: you need to find someone who has exactly what you want *and* wants exactly what you have. In a simple village economy with a few goods, this works. In a complex economy with millions of goods, it is hopelessly inefficient. The probability of a coincident match falls rapidly with the number of goods, creating enormous transaction costs that suppress trade. Money solves this by splitting every barter transaction into two: first, you sell your good for money; second, you use money to buy what you want from whoever has it, regardless of whether they want what you had. The double coincidence problem vanishes.
This is the medium of exchange function, and it is primary: it is why money exists. But for money to function as a medium of exchange efficiently, it needs the other two properties. The unit of account function means money serves as the common language for quoting prices. Without it, every pair of goods would need its own exchange ratio — with 1,000 goods, that is 499,500 distinct ratios. Money reduces this to 1,000 prices, all denominated in the same unit. This dramatically reduces the information problem in markets: buyers and sellers can compare prices across goods and across time because everything is measured on the same scale. Store of value — the ability to hold purchasing power over time — is what makes money useful for deferred exchange. If money were worthless tomorrow, rational agents would spend it immediately, creating hyperinflationary dynamics. A stable store of value allows people to earn income today and spend it later, enabling borrowing, saving, and intertemporal trade.
Historically, money evolved from commodity money (objects with intrinsic value: grain, cattle, gold, silver) to representative money (paper notes redeemable for a fixed quantity of a commodity, as under the gold standard) to fiat money (today's paper currency and bank deposits, backed only by government decree and collective trust). This evolution reflects a pursuit of better monetary properties: commodity money is scarce and credibly maintains value but is costly to produce and difficult to standardize; fiat money is cheap to produce and easily controlled but requires institutional trust to maintain value. The gold standard solved the store-of-value problem mechanically but constrained the money supply to gold production — a problem that amplified the Great Depression when falling prices made debts unpayable.
The M1/M2 aggregates you studied reflect a spectrum of liquidity — how easily an asset can be converted to a medium of exchange without loss of value. Currency is perfectly liquid. Checking account balances are functionally equivalent for most purchases. Savings accounts and small time deposits can be converted to cash quickly but with minor friction. As you move further out the liquidity spectrum, assets become better stores of value (earning interest, rising in price) but worse media of exchange. This is the fundamental monetary tradeoff: the safest, most liquid money earns no return, while higher returns come with less liquidity. The money supply definitions are ultimately about drawing a line on this continuum, and the appropriate line depends on the question being asked about the economy.