Since 1995, Derman has written many articles pointing out the essential difference between models in physics and models in finance. Good models in physics aim to predict the future accurately from the present, or to predict new previously unobserved phenomena; models in finance are used mostly to estimate the values of illiquid securities from liquid ones. Models in physics deal with objective variables; models in finance deal with subjective ones. “In physics there may one day be a Theory of Everything; in finance and the social sciences, you’re lucky if there is a usable theory of anything.”