“But there is this terrible thing in evil thoughts, that evil minds soon grow familiar with them.”
The narrator warns that repeatedly entertaining a harmful idea can reduce the resistance a person initially feels toward acting on it.
Source details
The Black Tulip, Chapter VIII, “An Invasion,” Project Gutenberg #965. Exact-witness review corrects the inherited Chapter VI locator: the narrator says that evil minds soon grow familiar with evil thoughts immediately after Boxtel first shrinks from informing against Van Baerle and before he rationalizes the act.