“The silent workings, and still more the explosions, of human passion which bring to light the darker elements of man’s nature present to the philosophical observer considerations of intrinsic interest; while to the jurist, the study of human nature and human character with its infinite varieties, especially as affecting the connection between motive and action, between irregular desire or evil disposition and crime itself, is equally indispensable and difficult.”—Wills on Circumstantial Evidence.