‘Papa! what’s money?’
The abrupt question had such immediate reference to the subject of Mr Dombey’s thoughts, that Mr Dombey was quite disconcerted.
‘What is money, Paul?’ he answered. ‘Money?’
‘Yes,’ said the child, laying his hands upon the elbows of his little chair, and turning the old face up towards Mr Dombey’s; ‘what is money?’
Mr Dombey was in a difficulty. He would have liked to give him some explanation involving the terms circulating-medium, currency, depreciation of currency, paper, bullion, rates of exchange, value of precious metals in the market, and so forth; but looking down at the little chair, and seeing what a long way down it was, he answered: ‘Gold, and silver, and copper, Guineas, shillings, half-pence. You know what they are?’
‘Oh yes, I know what they are,’ said Paul. ‘I don’t mean that Papa. I mean what’s money after all?’
From Dombey and Son by Charles Dickens.
Money is hard to define because it serves two separate functions. It is both a store of value and a measurement of value. It’s a bit like asking ‘what is a litre?’ You can buy milk by the litre and you can store a litre of milk in the fridge, and the meaning of ‘litre’ is different in each case.