Sometimes people ask me, “Doesn’t that make causal reasoning circular? Aren’t you just assuming what you want to prove?” The answer is no. By combining very mild, qualitative, and obvious assumptions (e.g., coat color of the son does not influence that of the parents) with his twenty years of guinea pig data, he obtained a quantitative and by no means obvious result: that 42 percent of the variation in coat color is due to heredity. Extracting the nonobvious from the obvious is not circular—it is a scientific triumph and deserves to be hailed as such.
