The unchanged lecture text, read in the winter semester of 1955/56 at the University of Freiburg i. Br., intentionally retains the repetitions of the same line of thought. The lecture was delivered on May 25, 1956, at the Club in Bremen and on October 24, 1956, at the University of Vienna. Unspoken content and subsequent notes are placed in square brackets. The principle of reason: Nihil est sine ratione. It translates to: Nothing is without reason. What this principle states is clear. It asserts that our understanding always and everywhere seeks a reason, or that human imagination necessarily always looks for a justification. According to Heidegger, this activity is older than the principle itself, which articulates it. More precisely, it was first formulated - in its Latin version - by Leibniz, thus in the 17th century. Positively expressed, the principle states: "Every being has a reason.".