This labor-saving and danger-avoiding device, moreover, proved so useful, that the idea soon suggested itself that by an extension of the principle involved in the bill of exchange the necessity of carrying gold at all in any quantity might also be avoided. A public office was therefore established, where people might deposit their gold under the guardianship of the state, and receive a ticket or receipt for the amount, payable in coin on demand; which tickets, from the fact that every body knew that they were convertible into gold at will, and that no more tickets were issued than corresponded to gold actually deposited and retained, soon came to be regarded as equally good and valid as gold itself, and vastly more convenient for the purpose of making exchanges. And thus it was that currency (from the Latin curro, to run) originated and came into use on the island as a substitute and representative of money. [15] The name originally given to these receipts was first "bank-credits," and then "bank-notes," but after a time people acquired a habit of designating them as "paper money." But this latter term was conceded to be but a mere fiction of speech and a bad use of language; for every intelligent person at once saw that a promise to deliver a commodity, or an acknowledgment of the receipt of, or a title to, a thing, could not possibly be the commodity or the thing itself, any more than a shadow could be the substance, or the picture of a horse a horse, or the smell of a good dinner the same as the dinner itself. Nevertheless, as an instrumentality for transferring commodities used for money, and avoiding the loss and waste unavoidable in handling and transporting such commodities, the currency thus devised was a great invention, and being always represented by, or, as we may express it, covered with, the commodity--gold--which, of all things, fluctuates least in value, it perfectly answered the purpose of money, without actually being so. It also furnished another striking illustration of the superiority of the commodity gold to serve either as money or as an object of value for deposit, against which receipts or certificates of deposit might be issued to serve as currency; for if other valuable commodities, like cattle, corn, cloth, or coal, had been selected for a like purpose, the bank would have been obliged to erect large pens, sheds, and warehouses for the storing of the deposits; and, let them be guarded ever so carefully, their value or purchasing power would, after a time, rapidly diminish from natural and unavoidable causes.