A monetary system requires first an abstract scale in which the value of different goods and services can be measured and compared, and secondly a transferable system of tallying or tracking credits for those supplying goods and services, and corresponding debts incurred by those consuming them. The essence of money does not therefore lie in gold, coinage, notes, or any other physical tokens themselves, but in what these things signify, which is the amount of these credits and debts. Thus in order to accept a simple bilateral IOU, a provider of goods or services must suppose that the buyer is trustworthy, and so can and will pay on demand, or according to an agreed schedule. But the seller may suppose instead, or in addition, that the IOU will itself be acceptable as credit by other people. In other words, the token and what it represents may be expected to be indefinitely transferable for a variety of goods and services at least within some circle of potential traders. Insofar as this is true, an IOU becomes liquid, that is, widely exchangeable, or in short a sum of money. The most liquid asset is a token of credit with the state or sovereign bank—commonly known as cash. The interest an institution pays on a security or a deposit can in effect be seen as a compensation for loss of liquidity, or equivalently the interest a borrower pays is the price of the credit that has been afforded by the lender.
Like other systems of mutual coordination a monetary system can arise more or less spontaneously (see convention). It is always as robust, or fragile, as the trust people feel able to put in its tokens of credit, and this is the question of whether they suppose, and suppose that others suppose, that the debts offsetting that credit are collectable. In a period of inflation (sometimes engineered by governments, sometimes not) money is devalued, which, as Adam Smith said has ‘always proved favourable to the debtor, and ruinous to the creditor’. A run on a bank is the result of a feedback loop whereby a suspicion that a bank’s own statement of credit cannot be made liquid grows and multiplies, thereby becoming self-justifying.