Oresme then proceeds to apply this central principle to the solution of the question which he sets himself to answer, and concludes that, as money is essentially a thing which exists for the public benefit, it must not be tampered with, nor varied in value, except in cases of absolute necessity, and in the presence of an uncontroverted general utility. He bases his opposition to unnecessary monetary variation on the perfectly sound ground that such variation is productive of loss either to those who are bound to make or bound to receive fixed sums in payment of obligations. The author then goes on to analyse the various kinds of variation, which he says are five--_figurae_, _proportionis_, _appellationis_, _ponderis_, and _materiae_. Changes of form (_figurae_) are only justified when it is found that the existing form is liable to increase the damage which the coins suffer from the wear and tear of usage, or when the existing currency has been degraded by widespread illegal coining; changes _proportionis_ are only allowable when the relative value of the different metals constituting the coinage have themselves changed; simple changes of name (_appellationis_), such as calling a mark a pound, are never allowed. Changes of the weight of the coins (_ponderis_) are pronounced by Oresme to be just as gross a fraud as the arbitrary alteration of the weights or measures by which corn or wine are sold; and changes of matter (_materiae_) are only to be tolerated when the supply of the old metal has become insufficient. The debasement of the coinage by the introduction of a cheaper alloy is condemned. In conclusion, Oresme insists that no alteration of any of the above kinds can be justified at the mere injunction of the prince; it must be accomplished _per ipsam communitatem_. The prince exercises the functions of the community in the matter of coinage not as _principalis actor_, but as _ordinationis publicae executor_. It is pointed out that arbitrary changes in the value of money are really equivalent to a particularly noxious form of taxation; that they seriously disorganise commerce and impoverish many merchants; and that the bad coinage drives the good out of circulation. This last observation is of special interest in a fourteenth-century writer, as it shows that Gresham's Law, which is usually credited to a sixteenth-century English economist, was perfectly well understood in the Middle Ages.[1] [Footnote 1: The best edition of Oresme's _Tractatus_ is that by Wolowski, published at Paris in 1864, which includes both the Latin and French texts.]