Nothing contributes so much to tranquilize the mind as a steady purpose, — a point on which the soul may fix its intellectual eye.
Mary Wollstonecraft (1759-1797) English social philosopher, feminist, writer
Frankenstein, Letter 1 (1818)
(Source)
More discussion of the history of this quotation: Nothing Contributes So Much To Tranquillize the Mind As a Steady Purpose,—a Point On Which the Soul May Fix Its Intellectual Eye – Quote Investigator