§5. _Some Consequences of a Higher Price Level_. The foregoing controversy will be of service to us, if it makes clear the manner and the spirit in which the marginal conception should be handled. It should be regarded not as a rigid formula which we can apply to diverse problems without considering the special features they present, but rather as a signpost which will enable us to find our way, a compass by which we may steer between the shoals of triviality and sophistry to the crux of any problem with which we have to deal. Let us illustrate its practical uses by an example which is of great interest and far-reaching practical importance at the present day. As has been already observed, the war has left behind it in all countries a great and almost certainly permanent increase in nominal purchasing power. Since the armistice prices have moved upwards and downwards with unprecedented violence; and it would be very rash to prophesy the precise level at which they will ultimately settle (using that word with considerable relativity). But, for reasons for which the reader is referred to Volume II in this series, it is safe enough to say that the general level of post-war will greatly exceed that of pre-war prices. Now this will apply not only to consumers' goods like milk and clothes, or to raw materials like pig-iron and cotton, but in very much the same degree to things like factories and machinery. Things of this last type are sometimes called "capital goods," because it is in them that a large part of the capital of a business is embodied. Now the fact that it will cost much more than it did before the war to construct fresh capital goods, has a significance which very few people appreciate. An existing factory cost, let us say, $500,000 to build and equip with machinery before the war. To construct a similar factory to-day would cost, let us assume (it is probably a moderate assumption) $1,000,000. Suppose 10 per cent to be the gross profit that is necessary to attract capital to the particular industry. Then it will not pay to construct this new factory unless the trade prospects point to the probability of a profit of about $100,000 per annum. But if the old factory is equally well managed, it too should be able to earn this $100,000, which upon the capital actually sunk would represent a rate of 20 per cent. The particular figures given are, of course, purely illustrative; the conclusion to which they point is that, if new enterprises are to be undertaken, pre-war enterprises are likely to yield a rate of profit, on their fixed capital at least, increased in rough proportion to the price-level. Of course, in years when trade is bad, the factory which dates from pre-war times will not earn a profit of this kind, it may very likely make an actual loss. At those times it is very certain that few new factories will be erected. But it is difficult to reconcile a condition of trade activity, in which the constructional industries