"Some of those bodies answer only to the deterministic laws of the mechanical philosophy," said Leibniz, "such as the globe, which rolled into the fireplace because her royal highness gave it a shove. But the bodies denominated Daniel Waterhouse, et cetera , are somehow different. True, they are subject to the same forces as the globe—our friend Daniel plainly feels Gravity’s pull, or else he would float away! But such bodies act in complicated ways not explainable by the laws set forth in your Principia Mathematica . When Dr. Waterhouse sits down to write an essay, let us say about the Latitudinarian philosophy espoused by him and the late Mr. Locke, we may observe his quill maneuvering all over the page in the most complicated paths imaginable. Here are none of the conic sections of the Principia ! No equation can predict the trajectory of Daniel’s nib over the page, for it results from innumerable and unfathomable minute contractions of the small muscles of his fingers and his hand. If we dissect a man’s hand, we find that these muscles are governed by nerves, which may easily enough be traced back to the brain, as rivers come from springs in the mountains. Remove the brain, or sever its connexions to the hand, and lo, that limb becomes as simple as yonder globe; that is, we may predict its future movements from the Principia, and plot them in Conics. And so it is evident that, to the Force of Gravity—which acts on everything—are superadded other forces, observable only in animals, * and productive of infinitely more complicated and interesting movements."