Gentzen's proof proves that one set of axioms we think is consistent proves that another set of axioms we think is consistent is indeed consistent. However both sets of axioms could be inconsistent. We don't think that is so, but it could be.
Gentzen's proof proves that one set of axioms we think is consistent proves that another set of axioms we think is consistent is indeed consistent. However both sets of axioms could be inconsistent. We don't think that is so, but it could be.