an object, of a surface, leads to a central point […]" Cézanne realized his painting ideas in the paintings of Montagne Sainte-Victoire and the his Still-Lifes. In his pictorial conception, even a mountain is understood as a superimposition of forms, spaces and structures that rise above the ground. Émile Bernard wrote of Cézanne's unusual way of working: "He began with the shadow parts and with one spot, on which he put a second, larger one, then a third, until all these shades, covering each other, modelled the object with their colouring. It was then that I realized that a law of harmony was guiding his work and that these modulations had a direction preordained in his mind.” In this preordained direction, for Cézanne, lay the real secret of painting in the context of harmony and the illusion of depth. To the collector Karl Ernst Osthaus Cézanne emphasized on 13 April 1906 during his visit to Aix that the main thing in a picture is the meeting of the distance. The colour must express every leap into the depths. Optical phenomena Cézanne was interested in the simplification of naturally occurring forms to their geometric essentials: he wanted to "treat nature in terms of the cylinder, the sphere and the cone" (a tree trunk may be conceived of as a cylinder, an apple or orange a sphere, for example). Additionally, Cézanne's desire to capture the truth of perception led him to explore binocular vision graphically, rendering slightly different, yet simultaneous visual perceptions of the same phenomena to provide the viewer with an aesthetic experience of depth different from those of earlier ideals of perspective, in particular single-point perspective. His interest in new ways of modelling space and volume derived from the stereoscopy obsession of his era and from reading Hippolyte Taine’s Berkelean theory of spatial perception. Cézanne's innovations have prompted critics to suggest such varied explanations as sick retinas, pure vision, and the influence of the steam railway. Aller sur le motif, sensation and realization Cézanne preferred to use these terms when describing his painting process. First of all, there is the “motif”, by which he not only meant the representational concept of the picture, but also the motivation for his tireless work of observing and painting. Aller sur le motif, as he called his approach to work, therefore meant entering into a relationship with an external object that moved the artist