History relates that on making the discovery he exclaimed: 'La natura è scritta in lingua matematica!' ('Nature is recorded in the language of mathematics.') These words reveal his surprise when he realized the implication of his discovery. Still, intuitively he must have known that using geometrical lengths to symbolize the measured magnitudes of forces would yield some valid result. Whence came this intuition, as well as the other which led him to recognize from the figures thus obtained that in a parallelogram made up of any two of the three lines, the remaining line came in as its diagonal? And, quite apart from the particular event of the discovery, how can we account for the very fact that nature - at least on a certain level of her existence - exhibits rules of action expressible in terms of logical principles immanent in the human mind?