Robert A. Heinlein Quotes about Hurt

Sin lies only in hurting others unnecessarily. All other "sins" are invented nonsense.
"Time Enough for Love". Book by Robert A. Heinlein, 1973.
"Stranger in a Strange Land". Book by Robert A. Heinlein, 1961.
Robert A. Heinlein (1987). “Stranger in a Strange Land”, p.295, Penguin
"Double Star". Book by Robert A. Heinlein, 1956.
Robert A. Heinlein (1987). “Time Enough for Love”, p.326, Penguin