'Knowledge of the Fixity of Law',
is a name for that 'Insight which is leading up' to the entrance into one of the 4 supermundane paths (vutthāna-gāmini-vipassanā).
In the Susīma-sutta (S.XII.70) this (ascending) Insight is called the 'Knowledge of the Fixity of the Law', namely: "At first, Susīma, there exists the knowledge of the Fixity of Law, and later the knowledge of Nirvana'" (See Vis.XXI).