And, in effect, communism (which has also had its metaphysical period, and earlier still a theological period) may, with entire justice, set against the two theses of free trade, two others of its own which consist: (_a_) in a different and not purely economic estimate of the maximum of social desirability; (_b_) in the assertion that this maximum can be attained, not through extreme free trade, but rather through the organisation of economic forces; which is the meaning of the famous saying concerning the _leap from the reign of necessity_ (=free competition or anarchy) _into that of liberty_ (=the command of man over the forces of nature even in the sphere of the social natural life). But neither can these two theses be proved; and for the same reasons. Ideals cannot be proved; and empirical calculations and practical convictions are not science. Pareto clearly recognises this quality in modern socialism; and agrees that the communistic system, as a system, is perfectly conceivable, _i.e._ theoretically it offers no internal contradictions (§ 446). According to him it clashes, not with scientific laws, but with _immense practical difficulties_ (_l.c._) such as the difficulty of adopting technical improvements without the trial and selection secured by free competition; the lack of stimuli to work; the choice of officials, which in a communistic society would be guided, still according to him, not by wholly technical reasons, as in modern industry, but on political and social grounds (837). He admits the socialist criticism of the waste due to free competition; but thinks this inevitable as a practical way of securing equilibrium of production. The real problem--he says--is: whether without the experiments of free competition it is possible to arrive at a knowledge of the line (the line which he calls _mn_) of the complete adaptation of production to demand, and whether the expense of making a unified (communistic) organisation of work, would not be greater than that needed to solve the equations of production by experiments (718, 867). He also acknowledged that there is something parasitical in the capitalist (Marx's _sad-faced knight_); but, at the same time, he maintains that the capitalist renders social services, for which we do not know how otherwise to provide.[58] If it be desired to state briefly the contrasts in the two different points of view, it may be said that human psychology is regarded by the free traders as for the most part, determined, and by the socialists, as for the most part changeable and adaptable. Now it is certain that human psychology does change and adapt itself; but the extent and rapidity of these changes are incapable of exact determination and are left to conjecture and opinion. Can they ever become the subject of exact calculation?