 Royce’s development of system \(\Sigma\) occurred before modal logic was formalized. While Royce uses the language of necessity and possibility in his discussion of logic, his system has no formal place for it. In fact, his student, C. I. Lewis, seeing the need to account for modal operators in understanding the conditionals of logic, proposed the first modal system in 1918, two years after Royce’s death. Lewis’s introduction of the strict conditional as a means of correcting Russell and Whitehead’s material conditional system required the introduction of modal operators to serve, in effect, as the parameter needed to connect the antecedent and consequent of conditionals (Pratt 2007).