The outlines of objects should not be sharp lines but should emerge from shadow like smoke (sfumato). Look at how distant mountains lose their definition, how the horizon blurs into sky. This is how reality appears to the eye, not with the hard edges of lesser painters.
Shadow is the absence of light; without it, there is no form. The painter must study how light falls on the sphere, the cylinder, the cone. All forms in nature can be reduced to these geometries. Master them, and you master the visible world.
Begin with drawing, for it is the foundation of all arts. Draw every day. Draw what you see, not what you think you see. Fill your workshop with bones, with dried plants, with casts of antique sculpture. Learn anatomy, for without it the figure is merely a sack. Study optics, for light is your medium. Above all, observe nature, for she is the true master.