If we want them to arrive at sound conclusions, a good many scales must be prepared for them, and these scales must be substantial, convergent, each with its own rungs of the ladder superposed, each with an indication of its total scope, each expressly designating the absent, doubtful, provisional or simply future and possible rungs, because they are in course of formation or on trial.[6215]--Consequently, these must all be got together in a designated place, in adjacent buildings, not alone the body of professors, the spokes-men of science, but collections, laboratories and libraries which constitute the instruments. Moreover, besides ordinary and regular courses of lectures, there must be lecture halls where, at appointed hours, every enterprising, knowledgeable person with something to say may speak to those who would like to listen. Thus, a sort of oral encyclopedia is organized, an universal exposition of human knowledge, a permanent exposition constantly renewed and open, to which its visitors, provided with a certificate of average instruction as an entrance ticket, will see with their own eyes, besides established science that which is under of formation, besides discoveries and proofs the way of discovering and proving, namely the method, history and general progress, the place of each science in its group, and of this group its place in the general whole. Owing to the extreme diversity of subjects taught there will be room and occupation for the extreme diversity of intelligences. Young minds can choose for themselves their own career, mount as high as their strength allows, climb up the tree of knowledge each on his own side, with his own ladder, in his own way, now passing from the branches to the trunk and again from the trunk to the branches, now from a remote bough to the principal branch and from that again back to the trunk.