or less accuracy according to the degree of interest; it is constantly given other directions by the change of external stimuli and by ideas. But, in spite of that, we are on the whole sufficiently able to see a house just when we want to see it and to receive practically the same picture of it ten times in succession in case no objective change has occurred.
There is nothing a priori absurd in the assumption that ordinary retention and reproduction, which, according to general agreement, is ranked next to sensorial perception, should also behave like it in this respect. Whether this is actually the case or not, however, I say now as I said before, cannot be decided in advance. Our present knowledge is much too fragmentary, too general, too largely obtained from the extraordinary to enable us to reach a decision on this point by its aid; that must be reserved for experiments especially adapted to that purpose. We must try in experimental fashion to keep as constant as possible those circumstances whose influence on retention and reproduction is known or suspected, and then ascertain whether that is sufficient. The material must be so chosen that decided differences of interest are, at least to all appearances, excluded; equality of attention may be promoted by preventing external disturbances; sudden fancies are not subject to control, but, on the whole, their disturbing effect is limited to the moment, and will be of comparatively little account if the time of the experiment is extended, etc.
When, however, we have actually obtained in such manner the greatest possible constancy of conditions attainable by us, how are we to know whether this is sufficient for our purpose? When are the circumstances, which will certainly offer differences enough to keen observation, sufficiently constant? The answer may be made:—When upon repetition of the experiment the results remain constant. The latter statement seems simple enough to be self-evident, but on closer approach to the matter still another difficulty is encountered.
Section 7. Constant Averages
When shall the results obtained from repeated experiments under circumstances as much alike as possible pass for constant or sufficiently constant? Is it when one result has the same
