Purnamaitrayaniputra, or Purna for short, is usually known as the disciple who was best at preaching the Buddha's teachings. He knew how to lecture according to the intelligence or the faculty for understanding of the audience. Thus, he could explain the Buddha's ideas in various ways so that people from all walks of life could understand the meaning of his teachings.

For example, when he lectured to medical doctors, he would remind them that they could treat patients' physical illnesses, but they could not cure their mental poisons of greed, anger and delusion. However, the Buddha's wisdom was like the pure honeydew that could help purify people's mental defilement. The Three Immeasurable Studies of precepts, meditation and wisdom proclaimed by the Buddha were the best medicine to cope with the Three Poisons.

When Purna lectured to judges, he told them that they could jail anyone who broke the law, but they could not prevent people from breaking the law in the first place because people's minds were still influenced by their environment. Only the Buddha's teachings, such as the law of cause and effect, could purify their minds to prevent them from doing evil.

He reminded farmers that they planted crops so people could have food to eat. However, they should also plant the field of blessings to nurture their wisdom. Planting the field of blessings meant believing in the Buddha's instructions, respecting the Three Precious Treasures of the Buddha, the dharma and the sangha, helping the sick, assisting one's parents, and not killing.

Whenever Purna lectured on the Buddha's teachings, he always kept the following ten guidelines in mind:
(1)comprehend the true meaning of the Buddha's teachings;
(2)be able to lecture about it;
(3)do not be intimidated by the crowd;
(4)be eloquent;
(5)lecture according to the ability of the audience to comprehend;
(6)carry out the teachings accordingly;
(7)be dignified;
(8)work diligently;
(9)do not show physical or mental exhaustion; and
(10)achieve great strength.
The Buddha therefore praised him for being the best in preaching, and he hoped that everyone else could learn from him.

Purna was from Kapilavastu. His father was King Suddhodana's preceptor. After his parents were married, his father went to a lakeside, worshipped Brahma, and asked for a child. That evening, he dreamed of various treasures flying into his wife's belly. Purna was born less than one year later. When Prince Siddhartha (who later became the Buddha） fled the palace to seek enlightenment, the whole country was surprised by the news. So was Purna, because although he had known the prince for years, he had not expected that the prince would leave behind his palace, father, wife, son, a luxurious life, and all his treasures. This came as a great blow to Purna. he pondered the prince's act, and finally he also left the palace in search of a good religious instructor. After some time, he went to the Buddha, became his disciple, and attained the arhathood.

One day, Purna and Maudgalyayana went to pay a visit to King Bimbisara of Magadha. The first king to become the Buddha's disciple, he was a devoted Buddhist. Later, the infamous Devadatta came to his country and incited the king's son. Ajatasatru, to usurp the throne and jail the king. The prince planned to starve his own father, the king, to death by prohibiting anyone from sending him food. However, the king's wife was able to feed him by smearing honey and cream over her body, visiting the king, and allowing the king to eat the honey and cream to stay alive. Unfortunately, the prince later discovered their stratagem, and he banned his mother from seeing the king.

The king believed that his son treated him this way because he had done something bad to his son in their previous lives, so he was now suffering from the effects of the causes that he had planted then. With this in mind, he felt very peaceful.

When the Buddha learned that the king was in jail, he sent Purna and Maudgalyayana to comfort the king. The two disciples entered the prison by means of their supernatural powers. "Your Majesty, the Buddha sent us to console you," they said to the king. "He would like you to know that our existence in the world is created by our own karma. We can eliminate our bad karma and obtain mental tranquility through spiritual cultivation."

"Anyone in jail has no freedom," they continued. "In the same way, no one in this world is free, because we are all shackled by money, prestige, power or the charm of beautiful women. Whether we are jailed or not, we are never totally free, so there is no need to be afraid of that. Whenever there is birth, there is ultimately death. Unfortunately, many people are reborn again in this world and once again they have to succumb to the power and the influence of bad karma, just like in their previous lives. The Buddha has said that one could recite the name of Amitabha Buddha and be reborn in the Pure Land." Hearing that, the king felt much more peaceful and relieved.