I wish to show how a variety of modes of thinking, easily recognizable in the progress of both the race and the individual, may be identified and arranged as successive species of the relationship which doubting bears to assurance; as various ratios, so to speak, which the vigor of doubting bears to mere acquiescence. The presumption is that the function of questioning is one which has continually grown in intensity and range, that doubt is continually chased back, and, being cornered, fights more desperately, and thus clears the ground more thoroughly. Its successive stations or arrests constitute stages of thinking. Or to change the metaphor, just in the degree that what has been accepted as fact--the object of assurance--loses stable equilibrium, the tension involved in the questioning attitude increases, until a readjustment gives a new and less easily shaken equilibrium. The natural tendency of man is not to press home a doubt, but to cut inquiry as short as possible. The practical man's impatience with theory has become a proverb; it expresses just the feeling that, since the thinking process is of use only in substituting certainty for doubt, any apparent prolongation of it is useless speculation, wasting time and diverting the mind from important issues. To follow the line of least resistance is to cut short the stay in the sphere of doubts and suggestions, and to make the speediest return into the world where one can act. The result, of course, is that difficulties are evaded or surmounted rather than really disposed of. Hence, in spite of the opposition of the would-be practical man, the needs of practice, of economy, and of efficiency have themselves compelled a continual deepening of doubt and widening of the area of investigation. It is within this evolution that we have to find our stages of thinking. The initial stage is where the doubt is hardly endured but not entertained; it is no welcome guest but an intruder, to be got rid of as speedily as possible. Development of alternative and competitive suggestions, the forming of suppositions (of ideas), goes but a little way. The mind seizes upon the nearest or most convenient instrument of dismissing doubt and reattaining security. At the other end is the definitive and conscious search for problems, and the development of elaborate and systematized methods of investigation--the industry and technique of science. Between these limits come processes which have started out upon the path of doubt and inquiry, and then halted by the way.