It is commonly known that C. I. Lewis started modern modal logics (S1-S5) because of his dissatisfaction with material implication. But his original intention was to construct a logic of strict implication rather than a logic of modality. Then he had got involved in the latter gradually against his original intention. In this paper, I clarify where his original intention lay and how he was involved in modal logics tracing his writings chronologically. Considering his contemporary logicians' responses, I sum up the argument.
