The ultimate test of scientific theory must be practice. If a theory aids in manipulating facts, if it leads to the discovery of ways of doing things which are better than old ways, if it solves problems which have hitherto remained unsolved, or carries the solution of problems farther than has hitherto been the case, it is a good theory. It need not be the best possible theory. It need not be a final theory. The chief claim for the present theory of value is that it not only unlocks all the doors that earlier theories have unlocked, but also others which have resisted the old keys. The man who goes into the modern stock market armed with marginal utility and the quantity theory is like the man who would fight Hindenburg with bows and arrows. Bows and arrows are effective in the hands of expert archers, and the great figures in the history of economics have done wonderful things with marginal utility, "real costs," and the quantity theory. But the social value theory is offered as a better weapon. The writer believes that the problem of the value of money has not been solved by the older theories of value. He believes that the social value theory will solve it. He proposes on the basis of the social value theory to make clearer the nature of credit phenomena, and to assimilate the laws of credit to the general laws of value. He proposes with the social value theory to bring together in a higher synthesis two divergent types of economic theory, the "static" and the "dynamic." He thinks that a rigorous and consistent application of the absolute concept of value will clarify confusions at various points in the general body of price theory, as the laws of supply and demand, etc. He offers the social value theory as the only way of giving a _psychological_ explanation to the demand-curve, and a marginal _value_ explanation of marginal demand-_price_. Demand-curves are social value curves, on the assumption of the fixed social value of the dollar. The utility theory, as will appear in the chapter on "Marginal Utility," has failed to give psychological magnitudes corresponding to _any_ point on the demand-curve. In general, he offers the social value notion as the justification for the assumption of a quantitative value which, as we shall see, underlies the whole of our current price analysis. The theory here outlined has been, as stated, developed and defended more fully in a previous book. For the rest, the author would have it judged by its usefulness or failure as a tool of thought in the investigations which follow.