It is suggested that analysis by means of "indifference functions" shall be extended to election among investment spaces; and the way in which that technique Could be developed to be applied to such problems is outlined in this work. The conditions of equilibrium, referred to the investor, are examined here. A necessary condition is that the proportional difference of the rates of return of resources be equal to the rate of space preference. If the structure of indifference functions renders the existence of a general equilibrium position possible, that equation must be verified for every investor simultaneously and, therefore, it defines a rate of space preference for the total of spaces considered. This market rate of space-preference permits to consider classic models of interregional trade, i.e., the "absolute costs" and the "comparative costs" models, as extremes of a continuum of situations.