Among the features that might characterize the XXth century - the one which begins in 1914-at least three are indisputable: in the political field, wars and revolutions which seem to defy all reason by the discrepancy between the mediocrity of men and the scope of the events, by the duration of their destructive momentum which no longer seems to be controlled by any rational intent, sometimes even by the active presence of some malignant will which becomes an end in and of itself; in the intellectual sphere, the separation of intellectual activity into varied disciplines which no longer have any necessary relation to each other, a specialization built upon the authority of that which we call science, however destructive of the organizing and integrating capacity of the human spirit; and finally in the spiritual realm, the sway of a temptation, that of bidding adieu to reason. Martin Heidegger, the greatest philosopher of the century, who for some years lent his authority to the national-socialist movement and who, disdaining any retractatio, ceaselessly denounced reason as "the most relentless enemy of thought", bears witness to this temptation with emblematic clarity.