The Theory of Money and Credit - Ludwig von Mises [27]
We may give the name commodity money to that sort of money that is at the same time a commercial commodity; and the name fiat money to money that comprises things with a special legal qualification. A third category may be called credit money, this being that sort of money which constitutes a claim against any physical or legal person. But these claims must not be both payable on demand and absolutely secure; if they were, there could be no difference between their value and that of the sum of money to which they referred, and they could not be subjected to an independent process of val uation on the part of those who dealt with them. In some way or other the maturity of these claims must be postponed to some future time. It can hardly be contested that fiat money in the strict sense of the word is theoretically conceivable. The theory of value proves the possibility of its existence. Whether fiat money has ever actually existed is, of course, another question, and one that cannot offhand be answered affirmatively. It can hardly be doubted that most of those kinds of money that are not commodity money must be classified as credit money. But only detailed historical investigation could clear this matter up.
Our terminology should prove more useful than that which is generally employed. It should express more clearly the peculiarities of the processes by which the different types of money are valued. It is certainly more correct than the usual distinction between metallic money and paper money. Metallic money comprises not only standard money but also token coins and such coins as the German thaler of the period 1873-1907; and paper money, as a rule, comprises not merely such fiat money and credit money as happen to be made of paper, but also convertible notes issued by banks or the state. This terminology is derived from popular usage. Previously, when more often than nowadays "metallic" money really was money and not a money substitute, perhaps the nomenclature was a little less inappropriate than it is now. Furthermore, it corresponded—perhaps still corresponds—to the naive and confused popular conception of value that sees in the precious metals something "intrinsically" valuable and in paper credit money something necessarily anomalous. Scientifically, this terminology is perfectly useless and a source of endless misunderstanding and misrepresentation. The greatest mistake that can be made in economic investigation is to fix attention on mere appearances, and so to fail to perceive the fundamental difference