In the same letter to Lister, Hartley writes that "a Man who disregards himself, who entirely abandons Self-Interest & devotes his Labours to the Service of Mankind, or in that beautiful and expressive phrase of the Scriptures, "who loves his neighbor as himself" is sure to meet with private Happiness". This conviction became a guiding principle in Hartley's life, and it led him to devote himself to a various philanthropic projects. These include the publication of Saunderson's "Elements of Algebra" and the promotion of the shorthand system of his friend John Byrom (a system that Hartley believed could be a "universal character" and step toward the creation of a philosophical language). Moreover, shortly after turning to medicine, Hartley became an advocate of variola inoculation for smallpox.