It is thus even up to the distinction between right and left which, far from being inherent in the nature of man in general, is very probably the product of representations which are religious and therefore collective.[12] Analogous proofs will be found presently in regard to the ideas of class, force, personality and efficacy. It is even possible to ask if the idea of contradiction does not also depend upon social conditions. What makes one tend to believe this is that the empire which the idea has exercised over human thought has varied with times and societies. To-day the principle of identity dominates scientific thought; but there are vast systems of representations which have played a considerable rôle in the history of ideas where it has frequently been set aside: these are the mythologies, from the grossest up to the most reasonable.[13] There, we are continually coming upon beings which have the most contradictory attributes simultaneously, who are at the same time one and many, material and spiritual, who can divide themselves up indefinitely without losing anything of their constitution; in mythology it is an axiom that the part is worth the whole. These variations through which the rules which seem to govern our present logic have passed prove that, far from being engraven through all eternity upon the mental constitution of men, they depend, at least in part, upon factors that are historical and consequently social. We do not know exactly what they are, but we may presume that they exist.[14] This hypothesis once admitted, the problem of knowledge is posed in new terms. Up to the present there have been only two doctrines in the field. For some, the categories cannot be derived from experience: they are logically prior to it and condition it. They are represented as so many simple and irreducible data, imminent in the human mind by virtue of its inborn constitution. For this reason they are said to be _a priori_. Others, however, hold that they are constructed and made up of pieces and bits, and that the individual is the artisan of this construction.[15] But each solution raises grave difficulties.