In the passage Brutus claim to Cassius that is he going to be able to see thing that are not there and Cassius replies that he will work as a mirror for him and help him to see the things he don’t see and support that he is honest and trustworthy telling Brutus several bad behavior that would make him doubt of Cassius and finish saying “then hold me dangerous” meaning that if Brutus see Cassius doing that things, only then he could think that Cassius is dangerous.
B on edg : Brutus suspects that Cassius’s flattery is fueled by intentions that will put him at risk, and Cassius assures Brutus that he is honest and trustworthy.
