In their turn, the individual psychical conditions become causes and effect, although with less power, the individual organic conditions and the issue of the struggle for life.  In the same way, the moral, juridical and political institutions, from effects become causes (there is, in fact, for modern science no substantial difference between cause and effect, except that the effect is always the latter of two related phenomena, and the cause always the former) and react in their turn, although with less efficacy, on the economic conditions.

An individual who has studied the laws of hygiene may influence beneficently, for instance, the imperfections of his digestive apparatus, but always within the very narrow limits of his organic capacities.  A scientific discovery, an electoral law may have an effect on industry or on the conditions of labor, but always within limits fixed by the framework of the fundamental economic organization.  This is why moral, juridical and political institutions have a greater influence on the relations between the various subdivisions of the class controlling the economic power (capitalists, industrial magnates, landed proprietors) than on the relations between the capitalist—­property-owners on the one side and the toilers on the other.

It suffices here for me to have mentioned this Marxian law and I will refer to the suggestive book of Achille Loria the reader who desires to see how this law scientifically explains all the phenomena, from the most trivial to the most imposing, of the social life.  This law is truly the most scientific and the most prolific sociological theory that has ever been discovered by the genius of man.  It furnishes, as I have already remarked, a scientific, physiological, experiential explanation of social history in the most magnificent dramas as well as of personal history in its most trivial episodes—­on explanation in perfect harmony with the entire trend—­which has been described as materialistic—­of modern scientific thought.[80]

If we leave out of consideration the two unscientific explanations of free will and divine providence, we find that two one-sided and therefore incomplete, although correct and scientific, explanations of human history have been given.  I refer to the physical determinism of Montesquieu, Buckle and Metschnikoff, and to the anthropological determinism of the ethnologists who find the explanation of the events of history in the organic and psychical characteristics of the various races of men.

Karl Marx sums up, combines and completes these two theories by his economic determinism.

The economic conditions—­which are the resultant of the ethnical energies and aptitudes acting in a given physical environment—­are the determining basis of all the moral, juridical and political phenomenal manifestations of human life, both individual and social.

This is the sublime conception, the fact-founded and scientific Marxian theory, which fears no criticism, resting as it does on the best established results of geology and biology, of psychology and sociology.