 Indeed, as we will see, a version of the Proof Interpretation is implicit already in Brouwer’s early writings from 1907 and 1908, and was notably used by him in his proofs of the bar theorem from 1924 and 1927, which predate Heyting’s papers on logic. We will therefore begin our account of the historical development of intuitionistic logic with Brouwer’s ideas, and then show how, via Heyting and others, the modern Proof Interpretation was arrived at.