Simplify the following text for a Grade 13 native English-speaking person.
Original: 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 ) .
Simplified: 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 ) .