Allow me to repeat what I said when you were here: deal with nature by means of the cylinder, the sphere and the cone, all placed in perspective, so that each side of an object or a plane is directed towards a central point. Lines parallel to the horizon give breadth, a section of nature, or if you prefer, of the spectacle spread before our eyes by the 'Pater Omnipotens Aeterne Deus'. Lines perpendicular to that horizon give depth. But for us men, nature has more depth than surface, hence the need to introduce in our vibrations of light, represented by reds and yellows, enough blue tints to give a feeling of air.

In: a letter to Émile Bernhard, 15 April 1904, as quoted in Letters of the great artists – from Blake to Pollock, Richard Friedenthal, Thames and Hudson, London, 1963, p. 180 - after 1900