The Nature of Force: Daughter of Motion, Mother of Weight
Force as offspring of spiritual and material motion, arising in muscle and nerve
Leonardo sets out his theory of force (forza), defining it as the daughter of material motion, granddaughter of spiritual motion, and mother and origin of weight; weight being finite while force is infinite. He names the four accidental powers - force, material motion, weight, and percussion - as those through which all mortal works have their being and their end. A further passage traces force to spiritual motion running through the limbs of sentient animals, thickening the muscles so that they shorten and pull the nerves joined to them. The transmitted-light sheet carries much additional untranscribed text in the right-hand column and a small marginal sketch at the lower right.
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Force, weight, and the four accidental powers
Force is the daughter of material motion, granddaughter of spiritual motion, and mother and origin of weight; the weight is finite in the elements of water and earth, while the force is infinite, since with it infinite worlds could be moved if instruments could be made in which it might be generated. Force with material motion, and weight with percussion, are the four accidental powers by which all the works of mortals have their being and their death.
How spiritual motion thickens muscles and pulls the nerves
Force takes its origin from spiritual motion, which running through the limbs of sentient animals thickens their muscles; the muscles being thickened come to shorten and draw back the nerves joined with them, and hence force is caused in the human limbs. The quality and quantity of a man's force can beget another force proportionally greater according to the length of its motion.