I found this:
Phase Two begins with the second edition of the book "Principal Problems of the
Theory of State Law" which was published in 1923. V. Kubeš allows that this phase
began probably a couple of years earlier when the original, exclusively static teaching,
primarily under the direct influence of Merkl, transformed into the dynamic legal
understanding. This stage sees relativization of former absolute opposites between the
world of reality or being (Sein) and the world of what ought to be (Sollen).http://facta.junis.ni.ac.rs/lap/lap2001/lap20...
and I found this:
In Kelsen’s theory, legal norms are essentially statements about what “ought to” be done. The concept of “ought to”
is part of Kelsen’s extensively discussed distinction between Sollen (“ought to”) and Sein (“is”) inspired by a
neokantian methodological distinction derived from Hume’s and Kant’s theory and is the core of his normativist
approach to law. Kelsen uses this distinction for separating the world of legal “normativity” from that of empirical
“facticity” which describes law as a matter of a fact or the actual behavior of men. See Hans Kelsen, Introduction to
the Problems of Legal Theory. A Translation of the First Edition of the Reine Rechtslehre or Pure Theory of Law
(Oxford: Oxford University Press 1992), 22–25.http://www.newschool.edu/uploadedFiles/NSSR/D...