 The lectures that Prof. Brouwer at the time held in Göttingen (for the first time) [1924 (van Dalen 2001: 305)], led me to the question how a Brouwerian propositional logic could be separated out, and I arrived at the result that this can be done by leaving out the single formula \(\neg \neg a \supset a\) (in your symbolism). I then also wrote to Prof. Brouwer [correcting Brouwer’s impression at the time that this formula is weaker than PEM]. (Troelstra 1990:  8)[17]