Since he swore an oath to uphold the Constitution, what he’s saying is that he doesn’t “know” if he’s not supposed to commit perjury.

And since it was already established with Clinton that perjury is an impeachable offense, what he’s saying is that he doesn’t grasp a very basic and straightforward legal reality of his office.

Which means he’s not of sound mind, and doesn’t even need to be impeached - he’s unfit for office and should be removed immediately.