Hi. I've reading "Type theory and formal proof, an introduction" book and in chapter 7.4 authors say, that in constructive logic plus either excluded third law (ET) or double negation law (DN) we can derive the other. And then authors derive DN from ET in calculus of constructions. But they didn't say anything (yet?) about the vice versa: deriving ET from DN in calculus of constructions. I've tried to do this, but the best i can come up with is just case analysis: - assume, that 'a' is true, then 'a or not a' is also true (by 'or-intro' rule). - or if after assuming that 'a' is true we can derive bottom, then 'not a' is true (by 'not-intro' rule). Then using the 'or-intro' rule we again end up with 'a or (not a)' being true. - assume 'not a' and if we can derive bottom again, then 'not (not a)' is true. Then by using DN we again end up with 'a' being true. etc. I.e. I may reduce any (not.. (n times) .. not a) into either 'a' or 'not a' by using DN and function composition. Thus, i probable can derive ET from DN using induction, but i can't code neither induction, nor above case analysis in calculus of constructions. So, is it possible to derive ET from DN in calculus of constructions? If it is, i'd appreciate not a direct answer, but a hint on how to do this. And if it is not, well, probably, authors will explain this later in the book and I should just continue reading. Thanks.