The priests and the doctrinaires of every sort have given themselves for centuries to this labor of abstraction, and have forced themselves to maintain the resulting illusions. Now that the positive sources of all ideologies have been found in the mechanism of life itself, we must explain realistically their mode of generation. And as that is true of all ideologies, it is true also and, in particular of those which consist in projecting ethical estimates beyond their natural and direct limits, making of them anticipations of divine announcements or presuppositions of universal suggestions of conscience. Therein lies the object of the special historic problems. We cannot always find the tie which unites certain ethical ideas to practical definite conditions. The concrete social psychology of past times often remains impenetrable to us. Often the commonest things remain for us unintelligible, for example, the animals considered as unclean, or the origin for the repugnance at marriage between persons of remote degrees of relationship. A prudent course of study leads us to conclude that the motives of many details will remain always concealed. Ignorance, superstition, singular illusions, symbolisms, these with many others are causes of that unconscious element, often found in customs, which now constitutes for us the unknown and the unknowable. The principal cause of all difficulty is precisely in the tardy appearance of what we call reason, so that the traces of the proximate motives of ideas have been lost or have remained enveloped in the ideas themselves. On the subject of science we can be much more brief. For a long time history has been made in an artless fashion. Granted and admitted that the different sciences have their statements in manuals and encyclopedias, it seemed sufficient to work out chronologically the appearance of the different formulas, resolving the total of the systematic summary into the elements which have successively served to compose it. The general presupposition was simple enough; underneath this chronology is the rational conception which develops and progresses. This method, if so it could be called, had within itself a certain disadvantage; it permitted us at best to understand how, one stage of science being granted, another stage of science may be derived from it by reason, but it did not permit us to discern by what condition of facts men were driven to discover science for the first time, that is to say, to reduce considered experience into a new and definite form. The question was, then, to find why there is an actual history of science, to find the origin of the scientific necessity, and what unites in a genetic fashion that necessity to our necessities in the continuity of the social _processus_.