Source identified1
“To be what we are, and to become what we are capable of becoming, is the only end of life.”
Stevenson’s formulation links self-acceptance with development: a worthwhile life is not imitation of someone else’s pattern but fuller realization of capacities that belong to one’s own character.
Source note: Familiar Studies of Men and Books, essay “Henry David Thoreau: His Character and Opinions,” section V; Project Gutenberg #425. The surrounding paragraph quotes several Thoreau statements, but this sentence is Stevenson’s own connective judgment, so it is not attributed to Thoreau.