[one watercolor] is in spite of all its difficulties, and perhaps because she has given so much trouble, less fresh and has become a little heavy - I… - Willem Roelofs

Ships, houses, mills... in one word everything that is made by people must stand upright and be painted with care. This is actually a good presentation compared to other, less symmetrical things, like the trees, skies, etc. It doesn't create the painting, but it certainly strengthen the illusion. It's just like somebody who is neatly dressed, but whose tie is coming off. The windows of a house must be straight, a mill in a pure construction, the blades well-positioned in perspective. (translation from original Dutch: Fons Heijnsbroek)

I certainly believe that the simple landscape which seems less impressive is the nature that is most proper to paint. (translation from original Dutch: Fons Heijnsbroek)

(original Dutch: citaat van Willem Roelofs, in het Nederlands:) Wij scheiden kleur en teekening af, omdat wij dat wel moeten. Maar de natuur doet dat niet. Zij geeft niet iets een vorm, om het daarna te kleuren. Vorm en kleur zijn inhaerente eigenschappen van het voorwerp, dat ons te schilderen is gegeven. Verwaarloozen wij een van beide, dan geven wij slechts de helft.