21. In any case, different answers to such questions yield different systems of modal logic . 22. He wrote important papers on free logic, general modal logic , and natural deduction systems. 23. S4 and S5 are the beginning of what is now called normal modal logic . 24. Pratt was an Assistant Professor at dynamic logic, a modal logic of structured behavior. 25. Semantic approaches dealing with modality are traditionally based on the principles of modal logic . 26. Epistemic logic is a modal logic dealing with the notions of knowledge and belief. 27. In a classical modal logic , each can be expressed by the other with negation: 28. The expansion rules depend on the particular modal logic used. 29. However, relatively few systems of modal logic can be formalised directly in natural deduction. 30. These methods have also been extended to modal logic .
