From the nightstand (Lust for Life; Irving Stone):

One day she ashed him, as he was preparing to attack
a new canvas, “How can you be sure that the spot you
choose will come out right on the canvas ?” 

Vincent thought for a moment and then replied, “If
I want to be active, I must not be afraid of failures.
When I see a blank canvas staring at me with a certain
imbecility I just dash something down.” 

“You certainly do dash. I never saw anything grow
as fast as your canvases.” 

“Well, I have to. I find paralysing the stare of a blank
canvas which says to me, ‘You don’t know anything!” 

“You mean it s a sort of challenge ?” 

“Exactly. The blank canvas stares at me like an idiot,
but I know that it is afraid of the passionate painter who
dares, who once and for all has broken the spell of that
’you cannot.’ Life Itself turns towards a man an infinitely
vacant, discouraging, hopelessly blank side on which
nothing is written, Margot, no more than on this
blank canvas.” 

“Yes, doesn’t it.” 

“But the man of faith and energy is not frightened by
that blankness ; he steps in, he acts, he builds up, he
creates, and in the end the canvas is no longer blank
but covered with the rich pattern of life..”