Leonardo is very explicit about this. Drawing is nothing but moving a point; not a craft but the gesture of a “light hand,” hardly leaving the trace of bodily activity behind — la pittura è mentale.
For Leonardo, without pittura all the artes would be impossible, since their codifications require visual signs. Painting – and that means, in this place, drawing, since color is not mentioned at all – not only provides letters for language, numbers for arithmetic, and figures for geometry; it also teaches perspectivists, astronomers, machinist, and engineers (questa insegna alli prospettivi et astrolaghi et alli machinatori et ingegneri). Drawing therefore is not only a media, conveying knowledge; it also appears as the original invention par excellence, giving birth to other human inventions. Consequently, it would be obsolete to insert painting into the system of the artes liberales.
Pittura claims a special rank, comparable, in a different context, only to theology.
Obviously, the theoretical claim that visual signs not only communicate, but generate knowledge, points to important epistemological decisions, harking back to the late medieval career of the Aristotelian phantasma. Per imaginem ad ideam: This reveals a clear succession, especially if the inner images are related, incontrovertibly to their origin, the senses. However, the praise of the minute, of speed, of the almost non-dimensional qualities of point and line, pen and drawing is related to older, Platonic paradigms. For instance, in Alberti’s and Leonardo’s apologia for the eye and the point, what is imprinted by the pen is itself a nusquam, leaving a trace when moved – the line.
The material medium of drawing of the drawing evaporates, and becomes a transitional being between mind and matter, analogous to the spiritus. Later on, this idea could be transformed into an argument against the sense and in favor of art as a domain of the mind–
-Frank Fehrenbach, The Pathos of Function: Leonardo’s Technical Drawings