  In spite of their fierce opposition, the two schools present elements of similarity alongside the numerous elements of difference. These have been recently debated among Neo-Kantianism scholars (Krijnen 2001, 77–93; Krijnen/Noras 2012). One indisputably common element between the two schools is the preoccupation with the problem of validity, or Geltung, which is arguably the systematic core of Neo-Kantianism in general (Krijnen 2001, 84). Philosophy for the Neo-Kantians is concerned with the systematic elucidation of the a priori principles that allow for valid thinking in various spheres of knowledge. In keeping with Kant’s first Critique, valid thinking is not merely logically consistent thinking. Valid thinking requires, over and above the adherence to the laws of formal logic, thinking in accordance with the a priori principles governing the intelligibility of a given area of reality. These will be, for instance, the principles of causality, substantiality etc. in the sphere of physical nature and principles such as good and evil, responsibility, freedom, etc. in the sphere of ethics. It is the task of philosophy to identify these a priori principles in all the different spheres of culture, including politics, biology, art, history, etc. Helmuth Plessner captures metaphorically the spirit of Neo-Kantianism when he characterizes the Neo-Kantians as “botanists in the garden of the a priori” (Plessner 1974, 185).