 As noted in  §3.2,  the deductive system SQML is sound and complete for SQML—all and only the logical truths of SQML are provable in SQML, including the three controversial principles BF, CBF, and \(\Box\textbf{N}\). Since, as we just saw in the previous section, those principles are all invalid in KQML, to formulate a sound and complete deductive system of his own, Kripke had to modify SQML rather severely to block the derivation of those principles without also blocking any of KQML’s valid formulas.