This paper presents first a general theory of a capitalistic optimum and a model illustrating its essential features, and, secondly, the empirical justification of this model, and its principal applications. Under very general conditions it is possible to show that we cannot expect, from an indefinite increase of available real capital, an indefinite increase of real national income consumed per inhabitant, and that there is an optimum amount of capital for which the real income per inhabitant is maximum. The conditions under which this maximum is attained are given. The general model, which is presented, and, in particular, its exponential variety, appear quite remarkably confirmed by all presently available empirical data, with respect to both the hypotheses and the results. A very simple expression of consumed real income is given in terms of the rate of interest i and the rate of growth Q.

The theories that summarize the state of our scientific knowledge are no doubt necessary vehicles for that knowledge. They should also provide support for further investigations of new ideas. But neither the theories nor the ideas are immutable: we must always be prepared to abandon, modify or replace them as soon as they no longer represent reality properly. In a word, theory must be adapted to fit nature: nature cannot be adapted to fit theory.... It follows that when a scientific idea has been put forward, the aim should not be to preserve it by seeking support for it at all cost, while discarding everything that might tend to invalidate it. The opposite should govern our behavior. The facts that seem to upset a theory should be examined with the greatest care, for real progress always consists of relinquishing an earlier theory which covers fewer fact.s by another of more extensive scope.