Abstract
Strictly speaking, intuitionistic logic is not a modal logic. There are, after all, no modal operators in the language. It is a subsystem of classical logic, not [like modal logic] an extension of it. But... (thus Fitting, p. 437, trying to justify inclusion of a large chapter on intuitionist logic ‘in a book that is largely about modal logics’).
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References
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Sylvan, R. Intuitionist logic — Subsystem of, extension of, or rival to, classical logic?. Philosophical Studies 53, 147–151 (1988). https://doi.org/10.1007/BF00355682
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DOI: https://doi.org/10.1007/BF00355682