Numerical regularities about disease, unknown in 1820, were commonplace by 1840. They were called laws, laws of the human body and its ailments. Similar statistical laws were gaining a hold over the human soul. The analogy was close, for laws of behaviour aimed at sick souls. Medical men were able to claim new expertise in matters moral and mental. Before proceeding, however, we should briefly ask an elementary question: what does a law of nature look like?

Our most familiar law is still Newton's. It says that the force of gravitational attraction between two bodies is equal to the product of their masses divided by the square of the distance between them — all multiplied by the gravitational constant. Newton did not write it that way, for he expressed his analysis in terms of ratios, so that the constant that we call ‘G’ is invisible. His work did imply a value for G. A 1740 French expedition to Mt Chimborazo in Ecuador made a fair experimental determination of it, but the observers thought of themselves as determining the mass of the earth.