In order to a have purpose in life, one must first discover…

“They say, Find a purpose in your life and live it. But, sometimes, it is only after you have lived that you recognize your life had a purpose, and likely one you never had in mind.”

Khaled Hosseini book And the Mountains Echoed

Source: And the Mountains Echoed

“Purpose of Life: The purpose of life is to live the life joyously, mindfully, peacefully and wisely.”

Thich Nhat Tu (1969) Vietnamese philosopher

Buddhist Socteriological Ethics: A Study of the Buddha’s Central Teachings (1999)

“I discovered, though, that once having given a pig an enema there is no turning back, no chance of resuming one of life's more stereotyped roles.”

E. B. White (1899–1985) American writer

"Death of a Pig," The Atlantic Monthly (January 1948)

“One must have felt a real desire to exchange thoughts with others in order to discover all that a lie can involve.”

Jean Piaget (1896–1980) Swiss psychologist, biologist, logician, philosopher & academic

Source: The Moral Judgment of the Child (1932), Ch. 2 : Adult Constraint and Moral Realism 
Context: One must have felt a real desire to exchange thoughts with others in order to discover all that a lie can involve. And this interchange of thoughts is from the first not possible between adults and children, because the initial inequality is too great and the child tries to imitate the adult and at the same time to protect himself against him rather than really to exchange thoughts with him. The situation we have described is thus almost the necessary outcome of unilateral respect. The spirit of the command having failed to be assimilated, the letter alone remains. Hence the phenomenon we have been observing. The child thinks of a lie as "what isn't true," independently of the subject's intentions. He even goes so far as to compare lies to those linguistic taboos, "naughty words." As for the judgment of responsibility, the further a lie is removed from reality, the more serious is the offense. Objective responsibility is thus the inevitable result of unilateral respect in its earliest stage.

“The purpose of life is not to be happy. It is to be useful, to be honorable, to be compassionate, to have it make some difference that you have lived and lived well.”

Ralph Waldo Emerson (1803–1882) American philosopher, essayist, and poet

Attributed to Emerson in Life’s Instructions for Wisdom, Success, and Happiness (2000) by H. Jackson Brown Jr., as well as numerous on-line sources since, the article "The Purpose of Life Is Not To Be Happy But To Matter" at the Quote Investigator https://quoteinvestigator.com/2014/11/29/purpose/ indicates that this quote is probably derived from various statements first made by Leo Rosten, including the following words delivered at the National Book Awards held in New York in 1962: "The purpose of life is not to be happy — but to matter, to be productive, to be useful, to have it make some difference that you lived at all."
Misattributed