Not less important as regards our attitude towards a given writer's statements is the purpose with which his book is written. The greatest confidence of course is inspired by books written with a purely scientific aim. Even the articles of observers who are not men of science are apparently much more carefully written if they are intended for purely scientific use in serious scientific journals (as some articles in the _Journal of the Ethnological Society_, _Jour. Anthrop. Inst._, etc.). Memoirs, descriptions of travel, and so on, give--_ceteris paribus_--less guarantee; often much more room is left to phantasy, to a tendency to amuse, perhaps puzzle or interest. Concrete instances of this could be easily adduced. At the end of all his mental operations, each observer had to generalize his observations, to express their common features, and formulate these in abstract and exact language. Here the most important points are personal intelligence and some mental training. The first is to be found even among the casual writers; for only people of a somewhat higher level of mentality would care to observe and write down their observations. But mental training in a scientific direction is exclusively to be found among the ethnographers; some of them stand far above all our other informants in matters of rather theoretical aspect, especially if social phenomena are concerned. And we may usually, in case of contradiction, take this information as the firm basis from which to start the operation of criticism. But on the other hand, there are reasons to mistrust general opinions laid down by professional ethnologists, for they are very often not simple generalizations, but theoretical inferences. Cases will be often met with where a general remark, which could be taken as a statement of fact--and often is given in such a form--appears after a more careful analysis to be quite a conjectural deduction from purely hypothetical premisses, or from incorrect definition. In all cases--_e. g._ where actual existence of group marriage is alleged--it will appear that this statement is a deduction from certain phenomena, which allow of quite a different interpretation, and that the term "marriage" is defined somewhat loosely.[35] [35] Compare also above, pp. 2-5.