on in the dark with the eyes shut. During the exercise of this faculty the investigator must in the most critical manner and with the aid of all necessary pertinent knowledge, think out or deduce everything that would happen, if an invented hypothesis were really true; and this he must do for each hypothesis in turn. The consequences appropriate to each hypothesis must be kept in groups by themselves; and these groups of hypothetical consequences must be carefully distinguished from the facts of observation.
In a geographical problem, the investigator must mentally search out, in view of each hypothesis that he has invented, the whole sequence of changes that would take place, the whole sequence of land forms that would be developed, if the class of forms with which he is dealing were followed all through its history, past, present and future. No invention should be hastily discarded, because it appears at first sight to be improbable; for such appearance may be more determined by the scientific fashion of the time, or by the mental habit of the investigator, than by anything inherent in the invention itself. Particular attention should be given to the deduction of unlike consequences of rival hypotheses; for, as will soon appear, it is particularly by means of these instantiæ crucis that successful and unsuccessful hypotheses are discriminated. Those who find deduction irksome should be advised to practise it until it becomes easy and agreeable; just as careless observers should be urged to continue observation until they can perform this fundamental process with accuracy and enjoyment. In no case should an investigator, particularly an unpractised investigator, put his trust in that rapid mental process called intuition, and hope by its uncertain aid to leap from invention to conviction. Let intuition be welcomed, just as invention is; but after it has leaped to its goal, its half-conscious path should be carefully retraced and the safety of its leap tested.
Confrontation of Consequences with Facts.—We now reach a stage in which the faculty of impartial comparison is brought into play. Facts have been gathered abundantly by the active observer, who is still at work gathering yet more of them; hypotheses have been brought forward in good number by the ingenious inventor, who is, however, still at work in the hope to find new ones; the consequences of each hypothesis have been carefully worked out, group by group, by the patient and logical deducer, who stands ready to elaborate the consequences of new hypotheses as soon as they are found; and the consequences are now to be confronted, group after group, with the facts by the impartial comparer, in order to see how close an agreement they reach. This is as if the observer should marshal his battallion of facts in good order on one side of a parade ground, and the deducer should lead forth the battalions of consequences one after another and halt them opposite the marshalled facts, so that the comparer could to best
