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I think what [the critic Gustave] Kahn says is quite true, that I haven’t paid enough attention to values, but it’ll be quite another thing they’ll say later — and no less true.
It’s not possible to do both values and colour.
Théodore Rousseau has done it better than anyone else, by mixing his colours [with bitumen] the darkness caused by time has increased, and now [some of] his paintings are hardly recognizable.
You can’t be at the pole and the equator at the same time. You have to choose. And I have high hopes of doing that, too, and it will probably be colour.
[SOURCE: Vincent van Gogh, Letter 594, addressed to Theo van Gogh, from Arles, Monday, 9 April 1888. Via vangoghletters.org.]