 Feys (1937) is the first to propose system T by subtracting axiom 4 from Gödel’s system S4 (see also Feys 1965: 123–124). In An Essay in Modal Logic (1951) von Wright discusses alethic, epistemic, and deontic modalities, and introduces system M, which Sobociński (1953) will prove to be equivalent to Feys’ system T. Von Wright (1951: 84–90) proves that system M contains Lewis’s S2, which contains S1—where system S is said to contain system S′ if all the formulas provable in S′ can be proved in S too. System S3, an extension of S2, is not contained in M. Nor is M contained in S3. Von Wright finds S3 of little independent interest, and sees no reason to adopt S3 instead of the stronger S4. In general, the Lewis systems are numbered in order of strength, with S1 the weakest and S5 the strongest, weaker systems being contained in the stronger ones.