Henri Matisse mentioned it in one of his letters:

It is only after years of preparation that the young artist should touch color– not color as description, that is, but as a means of intimate expression. Then he can hope that all the images, even all the symbols, which he uses, will be the reflection of his love for things, as a reflection in which he can have confidence if he has been able to carry out his education, with purity, and without lying to himself. Then he will employ color with discernment. He will place it in accordance with a natural design, unformulated and completely concealed, that will spring directly from his feelings; this is what allowed Toulouse-Lautrec, at the end of his life, to exclaim, “At last, I do not know how to draw anymore.”

The painter who is just beginning thinks that he paints from his heart. The artist who has completed his development also thinks that he paints from his heart. Only the latter is right, because his training and discipline allow him to accept impulses that he can, at least partially, conceal.

One of my co-workers ripped it off his Zen Calendar and gave it to me. I put it on the outside of my cubicle:

Lynda Barry on the power of the brush (via)