ReseñaPhilosophers have often enough set out to describe and examine the reasoning of mathematicians, the reasoning of experimental scientists, the reasoning of historians. I have attempted, in the same spirit, to describe and examine the reasoning of philosophers. But in such a way, I hope, as to avoid two common defects of methodological writings: insufficient illustration, and an undue anxiety to assimilate the varieties of rational procedure to a single type as if, for example, mathematicians never did anything but deduce conclusions from axioms, experimental scientists never reasoned except in the process of testing hypotheses, and historians wholly restricted themselves to the construction of narratives. I llustration, however, has its difficulties: any interpretation of a great philosopher is likely to provoke controversy. I have sometimes been obliged to ride roughshod over the finer feelings of scholars; I did not wish to interrupt the development of my general theme by defending in detail, or expounding in a less schematic way, the interpretations I have proposed. If this or that example is questioned, another may pass muster; a single illustration would suffice, logically, to make my point, since I am only concerned to show that certain reasoning-procedures do exist and are valid, not to establish their frequency. This is not a work of scholarship, although I have tried not to be unscholarly. My theme is such that I have been obliged to make ex parte pronouncements on a large number of philosophical topics. I have had to go out on a limb in order to survey a broader prospect. If the limb is rotten I must bear the consequences; and I know, at least, that I have not always proved it to be sound.