 The 1918 system takes as primitive the notion of impossibility \((\neg \Diamond)\), defines the operator of strict implication in its terms, and still employs an operator of intensional disjunction. However, Post will prove that this system leads to the collapse of necessity to truth—alternatively, of impossibility to falsity—since from one of its theorems \(((p\Rightarrow q)\Leftrightarrow(\neg \Diamond q\Rightarrow \neg \Diamond p))\) it can be proved that \((\neg p\Leftrightarrow \neg \Diamond p)\). In 1920, “Strict Implication—An Emendation”, Lewis fixes the system substituting the weaker axiom \(((p\Rightarrow q)\Rightarrow(\neg \Diamond q\Rightarrow \neg \Diamond p))\) for the old one. Finally, in Appendix II of the Lewis and Langford’s volume Symbolic Logic (1932: 492–502) “The Structure of the System of Strict Implication” the (weakened) 1918 system is given a new axiomatic base.