He combined this with a strong emphasis on the cultivation of in-depth insight into the doctrine of emptiness as taught by the Indian masters NÄ gÄ rjuna ( 2nd century ) and CandrakÄ `` rti ( 7th century ) . He combined this with a strong emphasis on the cultivation of in-depth insight into the doctrine of emptiness as propounded by the Indian masters NÄ gÄ rjuna ( 2nd century ) and CandrakÄ `` rti ( 7th century ) .