The following was taken from the introduction to Virginia Edition Volume No. 42. The excerpt is from a letter Heinlein wrote to his editor after the book was published in 1964.
This story is not properly science-fiction at all; the science-fictional aspects are mere devices and decoration. The story is an investigation of the nature of self-control and self-responsibility in a man who thinks of himself as “free,” and the nature of right conduct on the part of such a free individual in dealing with other individuals—when his own inner-directed nature requires that he extend to every individual (even his enemies) the same dignity and freedom that he demands for himself. One might say that the story is an investigation into the subtle but sharp distinction between “liberty” and “license.” Hugh Farnham is the archetype of the former, his son Duke is the archetype of the latter—yet each describes himself as a “free man.”