The most innovative aspect of Leonardo's technical contribution is his analysis of the components (the "organs") of machines, undertaken during the 1490s. He was the first to regard machines not as an indivisible whole, but as an assemblage of distinct parts. He was thus able to perceive that the infinite variety of machines derived from the many possible combinations of a finite number of mechanisms, which he defines as "elements of machines". Leonardo planned to devote an entire treatise to these elements, in which he would have broken down machines into their basic '"organs" and used quantitative methods to study their characteristics and performances. He 'espiants' the organs from the machines and depicted them in isolation from the other parts to which they were functionally connected.
During the same decade, Leonardo applied the same method to the study of the human body, whose organs he regarded as highly sophisticated mechanical devices. His vision of the anatomy of machines and man was enshrined in a series of masterly drawings that mark the birth of modern scientific illustration.
We shall discuss here the nature of the screw and of its lever, and how it [the screw] shall be used for lifting rather than for thrusting; and how it has more power when it is simple and not double, and thin rather than quick, when moved by a lever of the same length and force. We shall briefly discuss the many ways there are to use screws, the various types of endless [i.e. worm] screws, ... how the endless screw shall be combined with toothed wheels ... We shall examine the nature of their nuts ... We shall also speak of inverted screws and screws that by a single motion thrust and pull a weight, and screws which, by turning them only once, will rotate the nuts more than once. ... Their method of construction will also be discussed, how to put them to work ...
All such instruments will generally be presented without their armatures or other structures that might hinder the view of those who will study them. ... We shall also deal with the differences existing between a lever operating with constant force, that is, the wheel, and the lever of unequal power, that is, the straight beam, and why the former is better than the latter and the latter more compact and convenient than the former. We shall also discuss the ratchet wheel and its pawl, the flywheel and the impetus of the motion, the axles and their wear; ropes and pulleys, capstans and rollers, will also be described.
Introduction to the "Book of the elements of machines"
(Trattato degli elementi macchinali), Madrid Ms. I, fol. 82r)