Intuitionistic logic was founded by the Dutch mathematician L. E. J. Brouwer. He objected to "nonconstructive" proofs by contradiction. Many mathematicians have been sympathetic, but they were reluctant to use Brower's logic because it made many legitimate theorems more difficult to prove. For more info, see https://plato.stanford.edu/entries/intuitionism/ Re EGs: Peirce's diagrammatic reasoning, which begins with diagrams and reasons about them is by its very nature constructive. However, Peirce's rules of inference also allow proofs by contradiction. The idea of making EGs intuitionistic would enforce constraints on the rules of inference to prohibit the options that are nonconstructive. That could be considered as a kind of three-valued logic with True, False, and Not provable. But that's different from True, False, and Unknown -- or the fuzzy versions with a continuous range of values. Susan Haack strongly objected to Zadeh's idea of a continuum of truth values. But it would be more appropriate to consider the fuzzy values as degrees of confidence or belief. Re EGs: It's possible to translate any algebraic notation to and from an EG form. Therefore, any interpretation -- fuzzy, intuitionistic, modal, metalanguage -- that could be expressed in an algebraic form could be adapted to an EG version and vice-versa. By the way, Brouwer was a member of Lady Welby's Significs group, but it's unlikely that Peirce and Brouwer ever communicated. John
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