Vincent van Gogh letter to Theo van Gogh, Cuesmes, 24 June 1880:
In the springtime a bird in a cage knows very well that there’s something he’d be good for; he feels very clearly that there’s something to be done but he can’t do it; what it is he can’t clearly remember, and he has vague ideas and says to himself, "the others are building their nests and making their little ones and raising the brood", and he bangs his head against the bars of his cage.
And then the cage stays there and the bird is mad with suffering. "Look, there’s an idler", says another passing bird - "that fellow’s a sort of man of leisure".