There is a further difficulty, to which attention will be called more fully in later chapters, particularly the chapter on "Dodo Bones," and the chapter on the "Functions of Money." In other cases, in general, the capital value is, as the capitalization theory requires it to be, a true shadow, a passive function of the income and the discount, of the disk and the distance between the candle and the disk. In the case of money, however, the income is causally dependent, in part, upon the capital value. Money can function as money only by virtue of having value. The shadow becomes substance in the case of money. It is the _value of money_ which makes possible the _money work_. The capitalization theory, thus, if applicable at all, must be radically modified before being applied. We shall subsequently, in the chapters above referred to, take account of this fundamental complication. For the present, we can state it merely as a problem: how can we construe the interaction of the income value of money and the capital value of money in such a way as to avoid a circular theory? But further, the capitalization theory, as heretofore formulated, like the doctrines of supply and demand and cost of production, assumes _money_, and a _fixed absolute value_ of money. This assumption must be made if we are to be able to predict, on the basis of the capitalization theory, that a given annual income, at a given rate of discount, will give a specified capital value. This may be shown by the following considerations: If men anticipate that the value of the income, which is a fixed sum of dollars, is to grow less in the future, then the present worth of the bearer of that income will shrink to an extent greater than the "pure rate" of interest would call for. The principle of "appreciation and interest" comes in. The nominal interest, in times of falling value of money, tends to exceed the pure rate by an amount which compensates for the loss in value of future income as the dollar falls in value. We have here, however, a principle different from the principle of time discount. It is not the influence of time, which makes a _given_ value appear smaller as it is further removed in time, but it is an anticipated lessening in the value of the income itself, that counts. In terms of our candle and disk illustration, it is a factor affecting the size of the disk, rather than a factor affecting the distance between the disk and the candle. For the purposes of calculation, the two elements in the nominal rate of interest may be lumped together, and the nominal rate, rather than the pure rate, may be taken as the rate of discount for capitalization purposes. But for theoretical purposes, the two must be kept distinct. The capitalization theory rests on the assumption of a fixed value of the money unit.