What is the theme in A Scanner Darkly by Philip K. Dick?

The importance of animals and children is a theme of the book. Dick writes the novel with a great deal of empathy for the plight of his drug-abusing characters. This empathy, as he explains in the Author's Note, is because he was once a druggie himself. To highlight the innocence of his characters, Dick shows them reacting kindly to animals and children. After the druggies try to explain to a straight girl that she should not kill an insect because it is harmless, she says, "If I had known it was harmless I would have killed it myself."