What does Polonius show Claudius and Gertrude as proof that he knows the cause of madness?

Polonius shows Claudius and Gertrude love letters exchanged between Hamlet and Ophelia as proof that Hamlet's madness is actually caused by love. However, in reality, Hamlet's madness is not caused by love but rather by his grief and anger over his father's death and his mother's hasty remarriage.