qnli question: What are the three things John Herschel maintains need to be shown to allow for a mechanism be called a true cause? sentence: The structure of Darwin's argument showed the influence of John Herschel, whose philosophy of science maintained that a mechanism could be called a vera causa (true cause) if three things could be demonstrated: its existence in nature, its ability to produce the effects of interest, and its ability to explain a wide range of observations.